Please help me. Answer each letters and explain how you get the answer

Determined the number of girls and boys in each class.
a) There are 20 children in a class. 2/5 are boys
b) There are 42 children. 3/7 are girls
c) There are 15 children. The ratio of girls to boys is 3:2
d) There are 24 children. The ratio of girls to boys is 3:5