x - number of dimes, y - number of quarters
x + y = 11
0.10 x + 0.25 y = 1.70
0.10 x + 0.25 ( 11 - x ) = 1.70
you can solve it:
0.10 x + 2.75 x - 0.25 x = 1.70
0.15 x = 1.05
x = 1.05 : 0.15
x = 7 ( a boy has 7 dimes and 4 quarters )
Answer: 0.10 x + 0.25 ( 11 - x ) = 1.70
