The answer is 6. 4 quarters is 1 dollar right? and if you add 2 more, it will be $1.50 This is the easiest way to memorize how to multiply using the number 25.
Apply the AB rule, Adjust the fractions based on LCM, then you want to apply the a/b fraction rule which would allow you to do -15+23/24 which gives you 17/24. Hope this helps!