When 4 times a number is increased by 40, the answer is the same as when 100 is decreased by the num

When 4 times a number is increased by 40, the answer is the same as when 100 is decreased by the number. Find the number

The phrase a number, means an arbitrary variable, let's call it "x".

4 times a number, increased by 40, means we multiply 4 times x, and then add 40
4x + 40

100 decreased by the number means we subtract x from 100
100 - x

The problem tells us both of these expressions are the same, so we set them equal to each other:
4x + 40 = 100 - x

Add x to each side:
4x + x + 40 = 100 - x + x

The x's cancel on the right side, so we have:
5x + 40 = 100

Typing this equation into the search engine, we get x = 12.