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The Quest for 700: Weekly GMAT Challenge (Answer)

Yesterday, Manhattan GMAT posted a GMAT question on our blog. Today, they have followed up with the answer:

One way to break down this problem is to compare the quantities a pair at a time. First, consider 21/2 and 31/3. Write them next to each other, with a question mark between.
21/2 ? 31/3
The question mark stands for > or <, since one of them is almost certainly going to be larger. To compare these quantities exactly, we can raise them both to the same power, since their relative order will be preserved—that is, whichever one was larger to begin with will still be larger. (This works if both numbers are positive.)

Raising both numbers to the 6th power gets rid of both fractions in the exponents.
(21/2)6 ? (31/3)6
23 ? 32
Now we can easily evaluate: 23 = 8, whereas 32 = 9. This tells us that 21/2 < 31/3. (B) is larger than (A). Rule out (A).

Answer choice (C), 41/4, is the same as (22)1/4 = 21/2, which is the same as (A). So we can rule out (C) along with (A).

We can continue with pairwise comparisons by raising both expressions to a larger power that eliminates the fractional exponent.
Compare (B) and (D) by raising both to the 15th power.
(31/3)15 ? (51/5)15
35 ? 53
243 > 125
(B) is still larger.

Finally, compare (B) and (E) by raising both to the 6th power.
(31/3)6 ? (61/6)6
32 ? 61
9 > 6
(B) is still larger. Since it is larger than every other choice, it is the answer.

Alternatively, we could have raised every choice to the same large power (say, the 60th) to eliminate all the fractional exponents in one fell swoop. However, we would wind up with 230, 320, 415, 512, and 610, and we would still need to do some pairwise comparisons.

The correct answer is B.



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