Why can the mean, in statistics, be an estimate

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.

Ann and betty together have $60 ann has $9 more than twice betty’s amount how much money dose each have

What is the x-intercept and y-intercept of the equation 2x+1. 2o