Question 1012430
Call the number of dimes and quarters, d and q.  Then we have
d + q = 39  and the value equation is
10d + 25q = 645  (we work in cents)
Now multiply the top one by 10 and subtract from the second...
10d + 25q = 645
-(10d + 10q = 390)
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15q = 255
q = 17 quarters
and then we have 
d = 39 - q = 22 dimes