What is the least common multiple of 4 and 12?

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

Can someone me out here and the tell me the greatest common factor

Me. i can't figure out this question. give an explanation too. .