王刚家到县城有 x 千米.
(x - 6)/(x/3) + 6/(x/3 × 0.75) = 3 + 0.2
两边同乘以 5x,得
15(x - 6) + 120 = 16x
15x - 90 + 120 = 16x
x = 30 千米