I am thinking of a number. I multiply it by 14 and add 21. I get the same answer if I multiply by 4

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I am thinking of a number. I multiply it by 14 and add 21. I get the same answer if I multiply by 4 and add 141.

Let the number be n.

We have two expressions:

1. Multiply by 14 and add 21 is written as: 14n + 21
2. Multiply by 4 and add 141 is written as: 4n + 141

The phrase get the same expression means they are equal. So we set (1) and (2) equal to each other and solve for n:
14n + 21 = 4n + 141

Type this equation into our search engine to solve for n and we get:
n = 12