Let's call x to the unknown number
1. x*12 = 84
x = 84/12
x = 7
2. x - 8 = 9
x = 9 + 8
x = 17
4. 6/x = 9
6 = 9*x
6/9 = x
2/3 = x
5. x*7 = 175
x = 175/7
x = 25
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