If the sample MTBF is 525, should the manufacturer guarantee repair for failure up to 525 hours in the field?

Reliability Availability and Maintainability (RAM)
Assignment

Introduction
In this assignment, imagine that we've tested 10 electronic devices. For each device, the test was stopped at the occurrence of the fourth failure. Four devices experienced failure at 20, 40, 60, and 180 hours, respectively. Six devices ran 300 hours without failure.

Activity
Using the given scenario, complete the following:

Calculate the MTBF.
If the sample MTBF is 525, should the manufacturer guarantee repair for failure up to 525 hours in the field? Suppose the reliability function follows an exponential distribution. Explain your result and reasoning.
Use Excel or another appropriate tool to plot the reliability function R(t).
Conduct your own research on how to improve system reliability/sustainability, and explain your findings.