londonluddite wrote:

For each of her sales, a saleswoman receives a commission equal to 20 percent of the first $500 of the total amount of the sale, plus 30 percent of the total amount in excess of $500. If the total amount of one of her sales was $800, the saleswoman’s commission was approximately what percent of the total amount of the sale?

A. 22%

B. 24%

C. 25%

D. 27%

E. 28%

khan0210 wrote:

I have a question

Given that everyone has solved this,

if I do (19/80)*100 = 23.75% ~ 24%

Hence, choice B is the answer

if I round off 19 to 20 in order to get (20/80)*100=25%

Choice C is wrong.

So my question is, what is the way to ensure I am not falling into this rounding off calculation error?

Would you suggest not doing the rounding off. However, in other gmat questions rounding off is very helpful.

khan0210 , if answers are too close together, as here, do not estimate.

Determining what constitutes "too close together" is not an exact science.

But one and two

percent between answers?

\(\frac{1}{100}\) and \(\frac{2}{100}\)?

And hundreds of dollars at issue?

Don't round.

I agree that to estimate or round is often a shrewd strategy.

Experience with a lot of questions is the best teacher. I can say safely, however, that when the difference between and among answers is 1-2%, you should not estimate. See

Bunuel above. mikemcgarry wrote an excellent overview on the matter, titled

The Power of Estimation on GMAT Quant, here.Hope that helps.

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