Evaluate each expretion if a=2,b=3,and c=4 a+3b+8c

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

Restrictions on f(x) = 1/x+5 and g(x) = x-2

Icould really use some with this question.