How to work out questions like these, in detail

Let the missing number be x,

For the first question, the equation is as follows,
x - 13 = 29

Solve for x,
x - 13 + 13 = 29 + 13
x = 42

The equation would then be 42 - 13 = 29

For the second question, the equation is as follows,
x - 12 = 203

Solve for x,
x - 12 + 12 = 203 + 12
x = 215

The equation would then be 215 - 12 = 203

42-13=29 and

215-12=103

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