There are 72 boys. 7:6 = 84:?, divide 84 by 7, you get 12. Multiply 6 by 12 to get the answer.
The average can be calclculated as follows:
average = sum of number/number of numbers
average = (19+45) / 2 = 64/2 = 32
All you will have to do is put this inside of a graph and then see if it goes through the origin.
Hope This helps you..!!!!