Mental Math on the GRE

All of the following are calculations that could be done quicker using mental math than a calculator __except__:

Great! In general, long division is too tedious to be done by hand or mentally and thus should be done with a calculator. The only exception would be if you only need to obtain an approximate answer. On the other hand, the other calculations can be done fairly quickly by some reasoning and mental math: - $$\displaystyle \frac{4\sqrt{100}}{2}$$: This simple calculation should be done mentally for speed and also to avoid entry errors that are common when dealing with square roots within fractions. - The sum of the integers between -101 and 100: This can be calculated by noticing that all of the integers between -101 and 100 will cancel each other out when added, except for -101. Therefore, the sum is equal to -101. - 20% of 25,000: Whenever you come across percents that are easy to think of as fractions, you should always approach them with mental math. In this case, you know that taking 20% of a number is the same as taking one-fifth of that number, which is a simple calculation. - $$30^2$$: This can be computed simply by noticing that $$30=3\cdot10$$, so $$30^2=3^2 \cdot 10^2 = 9 \cdot 100 = 900$$.

Incorrect. This simple calculation should be done mentally for speed and also to avoid entry errors that are common when dealing with square roots within fractions.

Incorrect. This can be calculated by noticing that all of the integers between -101 and 100 will cancel each other out when added, except for -101. Therefore, the sum is equal to -101.

Incorrect. Whenever you come across percents that are easy to think of as fractions, you should always approach them with mental math. In this case, you know that taking 20% of a number is the same as taking one-fifth of that number, a calculation that is very simple.

Incorrect. This can be computed simply by noticing that $$30=3\cdot10$$, so $$30^2=3^2\cdot 10^2 = 9 \cdot 100 = 900$$.

$$\displaystyle \frac{4\sqrt{100}}{2}$$.

the sum of the integers between -101 and 100.

20% of 25,000.

$$56\overline{)24{,}543}$$.

$$30^2$$.