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Source: Official Guide for the GMAT 13th Ed. Problem Solving; #127 Official Guide for the GMAT 2015 14th Ed. Problem Solving; #127


If 3<x<100, for how many values of

If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

2 Explanations


my explanation>

Jan 30, 2015 • Comment


Gravatar Mike McGarry, Magoosh Tutor

Dec 28, 2013 • Comment

Michelle Dolan

This could be a silly question but why are we multiplying by 3 and not dividing by 3? I did get the same answer by dividing. This will help me learn from my mistakes as Kevin says. As always I love watching the explanations.


Oct 2, 2014 • Reply

Hi Michelle,

Happy to help! :) The reason that Mike decided to multiply by 3 in this problem was to make it a little easier. You can see in his t-chart that it's a little easier to take a prime number, square it, and then multiply by 3.

You can definitely leave the formula as it is and divide by 3 to solve the problem. This will still take you to the right answer as you discovered.

So I wouldn't say you made a mistake here. Rather you just chose a different path to the answer. And that's fine. I think the best thing to take away from this is that there are always multiple paths to the right answer. You need to find the fastest and the easiest for you. :D



Oct 9, 2014 • Reply

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