first you make sure you have all your variables using same measurement - cents (since it is the smallest)
$100 = 100*100 cents = 100,000 cents
1 quarter = 25 cents
1 nickel = 5 cent
1 dime = 10 cents
the test of your problem said that:
you have $100 in i don't know how many coins of quarters, nickels and dimes
100,000 = X * (25+5+10)
we have to find X:
100,000= X *(25+5+10)
100,000= X *40
X= 100,000/40
X = 2,500 coins
