Question 579154
Your equations are somewhat flipped:

{{{x+y = 35}}} ; total number of coins 

{{{.25x + .1y = 6.20}}} ; total value, x=quarters

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*[invoke linear "x", "y", 1, 1, 35, .25, .1, 6.20 ]

18 quarters, 17 dimes

CHECK:

18(.25) + 17(.10) = 4.50 + 1.70 = $6.20 &#8730;