There are twice as many nickels as dimes and five more dimes than quarters The total value is 6.40 How many quarters are there?

Twelve. Here is how: Using cents rather than decimal dollars: 5n + 10d + 25q = 640 Express the numbers of nickels and quarters in terms of the number of dimes: # of nickels = twice the # of dimes ==> n = 2d # of quarters = 5 less than the # of dimes ==> q = d-5 Substitute for n & q in original equation: 5(2d) + 10d + 25(d-5) = 640 45d - 125 = 640 45d = 765 d = 17 ==> 170 = $1.70 n = 34 ==> 170 = $1.70 q = 12 ==> 300 = $3.00 . . . . . . . . . Total = $6.40