How are numerical expressions and algebraic expressions different

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

What is best way to getting answer for my exam is

Negative 5 1/12 + negative 4 1/4 + 6 3/4