Question 1059797
d = # dimes
q = # quarters

d+q = 15               (1) (given)
10d + 25q = 240   (2) (given, expressed in cents)
 
10d + 10q = 150    (eqn(1) x10, call it (1'))
                15q = 90     (subtracted (1') from (2))
               q = 6       (divided both sides by 15)

Plug in q=6 into (1)  ((2) also would work but not as simple of an equation):  d=15-6=9

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Answer:  9 dimes, 6 quarters
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Check:   9+6=15  (ok on # of coins)
             10*9 + 25*6 = 90+150 = 240  (ok on value)