Let's look at what they give you...we don't know the number of quarters...but we have 4 times the number of dimes and 2 times the number of nickels...
quarters = .25x
dimes = 4(.10)x remember...4 times as many
nickels = 2(.05)x 2 times as many
this all equals $7.50
.25x + 4(.10)x + 2(.05)x = $7.50
.25x + .40x + .10x = $7.50
.75x = $7.50 (divide by .75)
x = 10 (this is how many quarters we have)
dimes = 4 times the quarters (4 x 10 = 40 dimes)
nickels = 2 times the quarters (2 x 10 = 20 nickels)
so, we have 40 dimes, 20 nickels, and 10 quarters