`a,`
`20 - 2^4 . x = 4`
`=> 2^4 . x = 20 - 4`
`=> 16 . x = 16`
`=> x = 16 : 16`
`=> x = 1
Vậy `x = 1`
`b,`
`8 . (x - 25) - 157 = 3^3 - 3^2`
`=> 8 . (x - 25) - 157 = 18`
`=> 8 . (x - 25) = 18 + 157`
`=> 8 . (x - 25) = 175`
`=> x - 25 = 175 : 8`
`=> x - 25 = 21,875`
`=> x = 21,875 + 25`
`=> x = 46,875`
Vậy `x = 46,875`
`c,`
`12x + 3x = 135`
`=> (12 + 3) . x = 135`
`=> 15 . x = 135`
`=> x = 135 : 15`
`=> x = 9`
Vậy `x = 9`