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.
What do you need help with?
I think you're right. I'm in pre-algebra, and sucks at it. Yet, I keep a A.
The answer is A. <span>Work backward.
Hong started talking with her friends at 10:10.
Hong started eating at 9:30.
Hong started shopping at 8:45.
Hong woke up at 8:25.</span>
I hope this helps you
if 18 gallons for 420 miles
? gallons for 357 miles
?.420=18.357
?=15,3