Logic: Thinking
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1. Bob and Carol and Ted and Alice either hate or love each of the other three people. The following is true:

Bob loves Ted and hates Carol.

Carol loves two of the other three people.

Carol hates everyone who hates her.

Only one person loves Alice.

Therefore, which must be true:

I. Ted hates Alice.
II. Ted loves Carol.
III. Ted loves Bob.

(A) none
(B) I, only
(C) II, only
(D) I and II, only
(E) II and III, only

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We know that Carol hates Bob right back (Carol is like that) so the two people she loves are Ted and Alice who love her back (otherwise, Carol would hate them). Carol is the one person who loves Alice which means Bob and Ted both hate Alice. So Ted hates Alice (I) is true and Ted loves Carol (II) is also true. We don't know how Ted feels about Bob so III is not true. The answer is (D).

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2. Bob and Carol and Ted and Alice are backpacking in the wilderness. Each person carries x pounds of equipment at the start of the trip. "Equipment" does not include food so the total weight of equipment (4x) is constant. During the first night, Bob sneaks 10 pounds of equipment from his pack into Carol's. Ted and Alice each sneak 7 pounds of equipment from their packs into Carol's. Later, when the other three are picking berries, Carol divides all the equipment in her pack evenly amongst the packs of her three pals. If Bob's pack now contains 54 pounds of equipment, how much did it contain originally?

(A) 32 pounds
(B) 35 pounds
(C) 40 pounds
(D) 42 pounds
(E) 45 pounds

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Bob starts with x pounds of equipment, gives some away and is left with (x–10) pounds. Meanwhile, Carol winds up with (x+10+7+7) = (x+24) pounds of equipment due to the "generosity" of her friends. Then Carol gives Bob (and Ted and Alice) one third of the equipment in her pack or (x/3 + 8) pounds. So Bob now has (x–10+x/3+8) pounds of equipment and we are told this is 54 pounds. Solving for x, you get 56 = (4/3)x, so x is 42 and the answer is (D).

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3. Bob and Carol and Ted and Alice smuggle treasure from a Caribbean island to the United States. The FBI is onto them. There are four possible outcomes and each of our four heroes will experience one outcome and each will experience a different outcome.

Bob or Carol or Ted will go to jail.

Ted or Alice will get rich.

Carol or Alice will change her identity.

Bob or Ted or Alice will go into hiding.

Which of the following is true?

I. If Bob goes to jail, Alice gets rich.
II. If Ted goes to jail, Bob goes into hiding.
III. If Carol goes to jail, Ted gets rich.

(A) III, only
(B) I and II, only
(C) I and III, only
(D) II and III, only
(E) I, II, and III

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Suppose Bob goes to jail. You could have Ted get rich, Carol change her identity, and Alice go into hiding. So (I) isn't true. Suppose Ted goes to jail. This means Alice has to get rich. If Alice gets rich, then Carol has to change her identity. This means Bob goes into hiding. So (II) is true. Finally, suppose Carol goes to jail. This means Alice has to change her identity. If Alice changes her identity then Ted must get rich. So (III) is true. The answer is (D).

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4. If ax2 + bx + c = 0 for all values of x, then which of the following is true?

I. a = b = c = 0
II. a > (b+c)
III. a2 > b2+c2

(A) none
(B) I, only
(C) II, only
(D) III, only
(E) II and III, only

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If a, b, and c are any non-zero numbers, you could solve for x using the quadratic formula and get 0, 1, or 2 solutions. The only way to make the equation true for ALL values of x (that's an infinite number of solutions including x = 0 and x = 1 trillion) is if a, b, and c are all zero. So the answer is (B).

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5. If n is an integer, for how many values of n does x2 + nx + n = 0 have integer solutions?

(A) none
(B) one
(C) two
(D) three
(E) more than three

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You might as well try a few values of n starting with zero to get a feel for this problem. If n=0 then x=0 that choice of n works. If n=1 you can't factor the quadratic so you aren't going to get an integer for x. If you want to factor the quadratic you need two numbers than both add to n and multiply to n. The only two numbers that multiply and add to the same thing are 2 and 2. So n can be zero or 4 and the answer is (C).

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6. If ab = cd and a > b > c > d then which of the following must be true?

I. a > 0
II. |b| = |c|
III. |c| < |d|

(A) I, only
(B) II, only
(C) I and II, only
(D) I and III, only
(E) II and III, only

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The only way to make the equation work is if a and b are both positive and c and d are both negative. For example: 4·3 = –2 · –6. So (I) and (III) are always true but (II) is not always true. The answer is (D).

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7. If line segment AB is parallel to the x-axis and M is the midpoint of AB and CM is perpendicular to AB, which of the following is sufficient additional information, by itself, to determine the area of triangle ABC?

I. the xy coordinates of points A and C
II. the xy coordinates (x,y) of points A and B
III. the xy coordinates of points C and M

(A) none
(B) I, only
(C) III, only
(D) I and III, only
(E) I, II, and III

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To get the area, you need the height and the base. If you are given the coordinates of A and C you can calculate the coordinates of the midpoint, M and the vertex B and then get the area. So (I) is enough information. Points A and B aren't enough however because you have no way to get the height. Points C and M aren't enough because you can't get the length of the base. So the answer is (B).

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8. Which of the following statements would, if true, provide sufficient additional information to determine the perimeter of the regular hexagon with center O shown in the figure?

I. The length of AO is 2 inches.
II. The length of BO is 2 inches.
III. The perimeter of triangle AOB is 2 inches.

(A) I, only
(B) II, only
(C) I or II, only
(D) I or III, only
(E) I or II or III

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The central angle of a hexagon is 60 degrees so the triangle is 30-60-90. If you have one side, you can get all three sides. Once you have the length of AB, you can easily get the perimeter of the hexagon. So AO and BO are obviously sufficient information. Knowing the perimeter of the triangle is also enough because the perimeter of a 30-60-90 with short leg x is just x + √3x + 2x so you could use the perimeter to solve for x. The answer is (E).

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9. A positive number x has three digits and ~x is defined as the number obtained by adding 1 to each digit that is less than 9 while leaving 9's unchanged. Which of the following must be true?

I. ~(~x) > ~x
II. ~x ≥ x + 100
III. 3 · x > ~x

(A) none
(B) I, only
(C) II, only
(D) III, only
(E) I, II, and III

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If you hit 999 with a squiggle, nothing happens so (I) and (II) aren't always true. The biggest change possible occurs if you hit 111 with a squiggle and change it into 222. Here, you've doubled the number. A squiggle will never triple the number so (III) is true.

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10. For a number n (n≠0), ‡n=1/n and *n=1–1/n. Therefore, for a number m (m≠0,1), *(‡(*(m)))=

(A) m
(B) ‡m
(C) *m
(D) ‡(‡m)
(E) *(*m)

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First star m to get (1 – 1/m) which is the same as (m-1)/m. Now hit this with a double dagger which just turns it upside down. So now you have m/(m-1). Hitting this with the star gives you 1 – (m–1)/m = (m–m+1)/m = 1/m or (B).

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