Binomial Distributions Question

Hi

Im having trouble doing this question.

Could someone help me out.

**A factory produces computer chips with a 0.9% defect rate. In a batch of 100 computer chips, what is the probability that**

a) only 1 is defective?

b) at least 3 are defective?

I guessed that p = 0.9 and q = 0.1 and n = 100

For a) I tried (100 Choose 1)*(0.9)^1 * (0.1)^99

but it turned out wrong..

The answers in the back of the book were a) 36.8% and b) 6.2%

How did they get these values?

Thanks in advance