If n represents the number, the product of the number and 9 is 9n. That value decreased by 10 is ...
9n -10
_____
Additional comment
The English expression is ambiguous in that we cannot tell if the number is decreased by 10 before becoming part of the product (9(n-10)), or if the correct interpretation is the one shown above. The order of operations is used as a guide, telling us that the multiplication is done before the subtraction since there is no indication of grouping.
