a, x + 5 = 20 - (12-7)
x + 5 = 20 - 5
x + 5 = 15
x = 15 - 5
x = 10
b, 4x - 20 = $2^{5}$ : 2²
4x - 20 = 2³
4x - 20 = 8
4x = 8 + 20
4x = 28
x = 28 : 4
x = 7
c, 75x + 49 . 28 = 199 . 28
75x = 199 . 28 - 49 . 28
75x = (199 - 49) . 28
75x = 150 . 28
75x = 4200
x = 4200 : 75
x = 56
d, $3^{x + 4}$ = 243
$3^{x + 4}$ = $3^{5}$
⇒ x + 4 = 5
x = 5 - 4
x = 1
e, 3636 : (12x - 91) = 36
12x - 91 = 3636 : 36
12x - 91 = 101
12x = 101 + 91
12x = 192
x = 192 : 12
x = 16
f, (x : 32 + 45) . 67 = 8911
x : 32 + 45 = 8911 : 67
x : 32 + 45 = 133
x : 32 = 133 - 45
x : 32 = 88
x = 88 . 32
x = 2816
Chúc bạn học tốt ^^