If d is the amount of dimes and the number of quarters is 8 times the number of dimes, then, you have 8d number of quarters.
Moreover, due to the total amount of money is $2.80, then, you have the following expression:
d + 8d = 2.80 simplify like terms left side
9d = 2.80
Hence, the equation is 9d = 2.80
To determine if your friend is correct, solve the equation for d by dividing by 9 both sides:
d = 2.80/9 =