Final answer:
The mathematical expression for 'five times a number y decreased by twelve' is 5y - 12.
Step-by-step explanation:
The expression for the quantity five times a number y decreased by twelve is 5y - 12. When a question asks for a mathematical expression of a verbal statement, it's important to identify the operations and the order in which they occur. 'Five times a number y' translates to 5y, and 'decreased by twelve' means to subtract twelve from that product, resulting in the expression 5y - 12.