Perimeter is adding all sides correct? Is the answer 54

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

Im literally having a mental breakdown

What are the greatest common factor of 75 and 30