Đáp án:
`a. (9+x)/(13 - x) = 5/6`
`(9+x) × 6 = (13 - x) × 5`
`9 × 6 + x × 6 = 13 × 5 - x × 5`
`54 + 6x = 65 - 5x`
`54 - 65 = -6x - 5x `
`-11 = -11x`
`-11x = -11`
`x = -11 : -11`
`x = 1`
`c. x + x : 5 × 75 + x : 2 × 9 = 315`
`1 × x + x × 1/5 × 75 + x × 1/2 × 9 = 315`
`1 × x + x × 15 + x × 9/2 = 315`
`(1 + 15 + 9/2) × x = 315`
`41/2 × x = 315`
`x = 315 : 41/2`
`x = 630/41`