by Tìm số tự nhiên x, biết:
a) 5x – 36 : 18 = 13
b) x – 105 : 21 = 15
c) (5x – 36) : 18 = 13
d) (x – 105) : 21 = 15
Hướng dẫn
a) 5x – 36 : 18 = 13
5x – 2 = 13
5x = 13+2
5x = 15
x = 15 :5
x = 3
Vậy x = 3
b) x – 105 : 21 = 15
x – 5 = 15
x = 15+5
x = 20
Vậy x = 20
c) (5x – 36) : 18 = 13
(5x – 36) = 13.18
(5x – 36) = 234
5x = 234 + 36
5x = 270
x = 270:5
x = 54
Vậy x = 54
d) (x – 105) : 21 = 15
(x – 105) = 15. 21
(x – 105) = 315
x = 315+ 105
x = 420
Vậy x = 420
