Assume that male and female births are equally likely and that the birth of any child does

Question:

Assume that male and female births are equally likely and that the birth of any child does not affect the probability of the gender of any other children. Find the probability of
a. Exactly 4 girls in 10 births.
b. At least 4 girls in 10 births.
c. Exactly 8 girls in 20 births.