=> (72 - x) . 9 = (x - 40) . 7
=> 648 - 9x = 7x - 280
=> 648 + 280 = 7x + 9x
=> 928 = 16x
=> x = 58
=> (72-x).9=(x-40).7
=> 648-9x=7x-280
=> 648+280-9x-7x=0
=> 928-16x=0
=> 928=16x
=> x=928:16
=> x=58
72-x/7=x-40/9
(72 - x) . 9 = (x - 40) . 7
<=> 648 - 9x = 7x - 280
<=> 648 + 280 = 7x + 9x
=> 928 = 16x
=> x = 58
Vậy x = 58