Originally Posted by

**Mahonroy** Hello,

I was working this problem, and was curious how I add the "10% of the forms" portion to the problem.

Here is the problem:

Among all the income-tax forms filed in a certain year, the mean tax paid was $2000 and the standard deviation was $500. In addition, for 10% of the forms, the tax paid was greater than $3000. A random samlpe of 625 tax forms is drawn.

a. What is the probability that the average tax paid on the sample form is greater than $1980?

So I did P(X > 1980)

X = N(2000,400)

Z = (1980 - 2000)/sqrt(400) = -1

Z-table gives me a value of 0.1587 for -1. So 1 - 0.1587 = 0.8413 probability. But how do you factor in the 10% thing?