What is the equivalent fraction for 15/18

Based on what i know about mean, this doesn't add up.

Which equation represents the model shown? a)1/3 divide 1/6 = 2 b)2/3 divide 1/6 = 2/18 c)1/3 divide 1/6 = 1/18 d)2/3 divide 1/6 =4

In a class, there are 12 girls. if the ratio of boys to girls is 5: 4 how many boys are there in the class