Question 776945
10D + 25Q = 640 (you see that a dime is 10 cents, and quarter, 25 cents)
D + Q = 40 (there are 40 coins) 

Multiply both sides of second equation by -10 to get

-10D - 10Q = -400 and add to first equation 

0D + 15Q = 240

Q = 16
D = 40-16 = 24

16 quarters are $4.00
24 dimes are  $2.40

$6.40 total