Let be the number of $2 coins Aaron has
Let be the number of $5 notes Aaron has
since we have 9 times as many $1 coins as $5 notes, we have:
since we have 8 times as many $2 coins as $5 notes, we have
the monetary value from the $1 coins will be $a dollars, the monetary value from the $2 coins will be $2b and the monetary value from the $5 notes will be $5c. Since Aaron has a total of $90, we have:
plug in the values for and from equations (1) and (2) respectively into equation (3), we get:
So, Aaron has 27 $1 coins, 24 $2 coins and 3 $5 notes. Adding these up, we get $90 total
EDIT: I seem to have misread the question, I found how many of each coin he had, oh well, it still gets the problem done, so i'll leave it. There is probably a way to solve for how many $2 coins there are without finding how many of the others there are, but i'm too lazy to think about that right now. (wait, not much thinking is required here, perhaps, we could have solved for the other two in terms of b and write equation 3 in terms of b as opposed to c and solve for b right off the bat. you can try that if you wish).