[email protected] wrote:Which of the following is equal to (0.9998)^2 ?

A) 0.99950014
B) 0.99950234
C) 0.99960004
D) 0.99960064

One option is to apply some algebra to this, but I think it's actually faster to recognize that we can perform a TINY PORTION of the multiplication and get the correct response.
Notice that the LAST TWO digits of the answer choices differ. So, we'll perform only the multiplication that affects those last two digits.


First, we'll multiply the 8 to get the following:

Notice that we didn't bother completing the entire multiplication.

Then, add with the customary 0 in the digits place and multiply the 9 to get the following:


IMPORTANT: since the next step will be to add 0's in the last two places before we begin multiplying, we know that all steps beyond this point WILL NOT affect the last 2 digits of the final answer. So, we'll stop here and add our products to get the following:


As you can see the correct answer is C

Cheers,
Brent

This question illustrates the importance of checking the answer choices BEFORE choosing a specific approach. For example, if answer choices looked something like this...
A) 0.99890004
B) 0.99950004
C) 0.99951004
D) 0.99960004
...I might have used an approach different from the one I chose.

Aside: The importance of ALWAYS checking the answer choices BEFORE choosing a specific approach is covered in our free video on General GMAT Math Strategies: https://www.gmatprepnow.com/module/gener ... es?id=1111

Cheers,
Brent