CRE Handbook2023 - Example 2.3 - Question for MTBF

Hello everyone,


I would like to know what is the formula used to find the value for MTBF?
I applied the data obtained in all formulas presented in the book and the value is not the same.

Moreover, I would like to understand, because the statement says that the parts are not recoverable, so it would not be correct to use the formula MTTF instead of MTBF?