3/4 with the main number divide it
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:
4
Step-by-step explanation:
Answer:
x = 13
Step-by-step explanation:
This question is based on Secant Secant theorem.
Secant Secant theorem gives us the following formula:
(AB + BD)AB = (AC + CE).AC
From the above question we have the following parameters
AB = 5
BD = x
AC = 7.5
CE = 4.5
Hence,
(AB + BD)AB = (AC + CE).AC
(5 + x)5 = (7.5 + 4.5)7.5
25 + 5x = 90
Collect like terms
5x = 90 - 25
5x = 65
x = 65/5
x = 13