x=quarters y=dimes
x+y=74
.25x+.10y=15.89
then substitute one equation into the next so...
y=74-x
.25x+.10(74-x)=15.89
.25x+7.4-.10x=15.89 then simplify
.15x=8.49
x=56.6 round up so 57 quarters
to find y just plug in x so...
57+y=74
y=17
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