Please solve math problem for me... I will mark you brainliest

If f(x)=3x+1 and f^-1=x-1/3, then f^-1(7)=

Some boys and girls were asked if they had a pet. one third of the group were boys. there were 20 boys. 20% of the girls had a pet. boys with a pet : girls with a pet = 1 : 5 i need to know how many girls there are.

Ineed to know the answer to this algebra question.