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Bunuel
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Three of the sides of a rectangular prism have areas of 91, 39, and 21. What is the volume of the rectangular prism?


Multiply these three: \((abc)^2 = 91*39*21 = (7*13)*(3*13)*(3*7) = 3^2*7^2*13^2\) --> \(abc=3*7*13=272\).

Answer: C.

Hi Bunuel,
I think there is a typo, the answer would be 273, not 272 :) :)
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jamesphw
Three of the sides of a rectangular prism have areas of 91, 39, and 21. What is the volume of the rectangular prism?

A) 252
B) 269
C) 273
D) 920
E) 1911

One easy way to get the answer for this question is just multiply the unit's digits of these three numbers, i.e (1*9*1 = 9) (Why the multiplication is needed?, you can see the solution suggested by Bunuel in the above post)
Now look out for the answer whose square gives us a 9 in the unit place.
So, it is easy to pic (C) 273 whose square will give us a 9 in the unit place( 3*3), and hence this is the answer.

Hope that helps..
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Bunuel
jamesphw
Three of the sides of a rectangular prism have areas of 91, 39, and 21. What is the volume of the rectangular prism?


Multiply these three: \((abc)^2 = 91*39*21 = (7*13)*(3*13)*(3*7) = 3^2*7^2*13^2\) --> \(abc=3*7*13=272\).

Answer: C.

Hi Bunuel,
I think there is a typo, the answer would be 273, not 272 :) :)

Typo edited. Thank you.
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jamesphw
Three of the sides of a rectangular prism have areas of 91, 39, and 21. What is the volume of the rectangular prism?

A) 252
B) 269
C) 273
D) 920
E) 1911


A great example of a question where doing a bunch of math gets you to the right answer...but so does just using a little logic...logic is faster, easier, and gets us to the same destination, so I don't understand why we'd bother with the math. (See my signature.)

Sure, the lengths could be non-integers, but c'mon, look at the numbers...21 is 3*7...39 is 3*13...those two are obvious...if 91 is 7*13, we're all good...sure enough!!
So the volume is 3*7*13 = 21*13 = 260+13 = 273

Answer choice C.
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