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For the positive integers a, b, and k, ak||b means that ak is a divisor of b, but ak+1 is not a divisor of b. If k is a positive integer and 2k||72, then k is equal to

For the positive integers a, b, and k, \( a^k \parallel b \) means that \( a^k \) is a divisor of b, but \( a^{k+1} \) is not a divisor of b. If k is a positive integer and \( 2^k \parallel 72 \), then k is equal to

A. 2

B. 3

C. 4

D. 8

E. 18

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Correct Answer: B

You’ll find tons of practice questions, explanations for GMAT Official Guide questions, and strategies on our GMAT Question of the Day page.

Here are a few other extra challenging GMAT questions with in depth explanations:

Here’s a tough function question from the GMAT Prep tests 1 and 2:

For which of the following functions is f(a+b) = f(b) + f(a) for all positive numbers a and b?

And a very challenging word problem from the Official Guide. Almost no-one gets this one on the first try but there is a somewhat simple way through it:

Last Sunday a certain store sold copies of Newspaper A for $1.00 each and copies of Newspaper B for $1.25 each, and the store sold no other newspapers that day. If r percent of the store’s revenues from newspaper sales was from Newspaper A and if p percent of the newspapers that the store sold were copies of newspaper A, which of the following expresses r in terms of p?

Tanya’s letters from the GMAT Prep tests. This one often gets GMAT tutoring students caught up in a tangled net. With combinatorics it’s important to stay practical. We’ll take a look at how to do that in the explanation:

Tanya prepared 4 different letters to be sent to 4 different addresses. For each letter, she prepared an envelope with its correct address. If the 4 letters are to be put into the 4 envelopes at random, what is the probability that only 1 letter will be put into the envelope with its correct address?

Here’s an exponents puzzle that comes up a lot in GMAT tutoring sessions:

If n is a positive integer and n^2 is divisible by 72, then the largest positive integer that must divide n is

This is one of the most difficult questions in the GMAT universe. That said, there is a simple way to solve it that relies on a fundamental divisibility rule every GMAT studier should know:

For every positive even integer n, the function h(n) is defined to be the product of all the even integers from 2 to n, inclusive. If p is the smallest prime factor of h(100) +1, then p is?