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I believe its 2 and 1/2
I'm so sorry if its not but I would guess it is :)
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.
Answer:
73
Step-by-step explanation:
48^2 + 55^2 = 5329
square root of 5329 is 73