x = number of quarters
y = number of half
x + y = 26 => y = 26 - x
x * $0.25 + y * $0.5 = $11.50
so
x * 0.25 + (26 - x) * 0.5 = 11.5
x * (0.25 - 0.5) = 11.5 - 26*0.5
x * (-0.25) = 11.5 - 13 = -1.5
x = (-1.5)/(-0.25) = 6
He have 6 quarters and 20 half dollars
