Bài 3:
a) X x 6 = 837 - 747
X x 6 = 90
x = 90 : 6
x = 15
b) X : 7 = 12 (dư 6)
X = 12 x 7 + 6
X = 90
c) X : 4 + 135 = 219
X : 4 = 219 - 135
X : 4 = 84
X = 84 x 4
X = 336
d) 824 - 12 x 9 < X x 4 + 625 < 493 + 75 x 3
824 - 108 < X x 4 + 625 < 493 + 225
716 < X x 4 + 625 < 718
Vì 716 < 717 < 718
Vậy X x 4 + 625 = 717
X x 4 = 717 - 625
X x 4 = 92
X = 92 : 4
X = 23