Identify the common external tangent

There are 20 children in the cast of a class play and 8 of the children are boys of the boys 4 have a speaking part in the play and of the girls 8 do not have a speaking part in the play if a child from the cast of the place chosen at random what is the probability that the child has a speaking part

Find the domain of the following expression: 1/(√x)-2

Wrote and expression for the difference of p and 4