Binomial question

Hi I need help in the following question

10% of all doctors recommend a new headache medicine. If we randomly select 20 doctors(assume Independence), find the probability at least 3 recommend the medicine (2)fewer than 4 recommend the medicine

Please help.Thank you very much

1) Find the probabilities that 0, 1, or 2 (exactly) make the recommendation. Sum these probabilities. Subtract the sum from 1.

2) Find the probability that exactly 3 make the recommendation. Add this to the previous sum.

:wink:

hopefully that cleared it up,
but if not, do you know how to use the "binomial expansion" ?

In the sample of 20 docs,
1 recommended it means 1 did and 19 didn't and so on.
So that's where the indices in the expansion come from.

Can you work out the p and q, knowing q is 1-p ?