N + D =31

5N+ 10D = 255

5N + 5 D = 155

5D =100

D=20 dimes

N= 11 nickels

20+11 = 31 coins

each nickel is worth 5 cents 11 nickels = 55 cents

each dime is worth ten cents 20 dimes = 2 dollars

2 dollars plus 55 cents = $2,55

20 dimes plus 11 nickels = 31 coins

or

5N +10D =255 divide by 5

N +2D = 51

N + D =31 subtract

2D- D = 51-31 = 20

D=20 dimes

N=31-20 = 11 nickels