X : 380 =400
X = 400×380
X = 152 000
152 000 : 380 = 400
X × 6 = 720
X = 720 : 6
X = 120
120×6 = 720
850 : X = 50
X = 850 : 50
X = 17
850 : 17 = 50
900 × X = 3600
X = 3600 : 900
X = 4
900 × 4 = 3600
964 + X = 1020
X = 1020 - 964
X = 56
964 + 56 = 1020
X - 2871 = 624
X = 2871 + 624
X = 3495
3495 - 2871 = 624