Answer :
5c-100 since she has 100 dollars then every time she buys a charm she subtracts from the 100 and you multiply 5 by the total amount of charms she bought.
Amount of money she has left is 100 - 5x
Number of charms she buys is $5x
x being the number of charms
Number of charms she buys is $5x
x being the number of charms