Answer: 1) Each friend had contributed $6.25.
2)The expression to show how many books Aaron read in p months :

He reads 54 books in 6 months.
Step-by-step explanation:
1) Given : The total number of friends who contributed equal amounts of money to buy a present. : 4
The total amount they have = $25
Since all the friends contributed the same amount.
Thus , the amount each had contributed :

Hence, each friend had contributed $6.25.
2) Given : The number of books Aron reads each month = 9 books
Let the number of months be 'p'.
Then the expression to show how many books Aaron read in p months .

Then , the number of books Aaron read if he read for 6 months.

Hence, he reads 54 books in 6 months.