You are asked to find the missing value in
... 40 + ? = 70
given that it is also the solution to
... ? + 5 = 35
Either way, you subtract the constant that is on the same side of the equation as the unknown.
... 40 + ? - 40 = 70 - 40
... ? = 30 . . . . the missing value
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You can check to see if this also satisfies the equation
... 30 + 5 = 35 . . . . . . . true
