the product of 8 and 15 more than a number

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the product of 8 and 15 more than a number.

Take this algebraic expression in pieces.

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

15 more than x means we add 15 to x:
x + 15

The product of 8 and 15 more than a number means we multiply 8 by x + 15
8(x + 15)