They are essentially asking us to solve:

Solving this equation for x tells us that it will take around 80.1 years for the materials in the vault to reduce to 357 pounds.
The answer is 18. You can find the mean by adding the numbers together, then dividing the sum by the amount of salads (or other variable, in this case, 5.)
Answer:
a
Step-by-step explanation:
they are right