A student has $1 bills and $5 bills in his wallet. He has a total of 15 bills that are all worth $47. How many of each type of bill does he have?

let number of  $1 bill = x

so $5 bill number = 15-x

now

x*1 + (15-x)*5 = 47

4x= 28

x=7

so number of $1 bill = 7

no of $5 bill = 15-7 = 8