What is y in 3y+1+12y-4=75

Find the value of the cosine for angle a

Icould really use some with this question.

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.