緊急!!數學!!!! 10分!!!!

設$x 為他原來每星期的入息
x(1 + 20%)(1 - 10%) = x + 320
x * 1.2 * 0.9 = x + 320
1.08 x = x + 320
(1.08 - 1) x = 320
0.08x = 320
x = 320 / 0.08 = 4000
小明原來每星期的入息為$4000

(100%+20%)-{(100%-20%)x10%}
=120%-12%
=8%

$320/8%
=$40

$40x100
=4000