Final answer:
The new average after multiplying each number by 9 is 162, which is obtained by multiplying the original average, 18, by 9.
Step-by-step explanation:
To find the new average after each number in a set is multiplied by a constant, we can simply multiply the original average by that constant.
In this case, the original average of the 15 numbers is 18.
When we multiply each number by 9, we also multiply the average by 9.
Therefore, the new average is 18 × 9, which is 162.