Please help - I don't get it!?

Question

The standard deviation of the lifetime of washing machines is estimated to be 800 hours.

a. What value of z would be used to achieve a 97% two tail probability?

b. How large a sample must be taken in order to be 97% confident that the margin of error will not exceed 50 hours?

Please show how you arrived at all answers. Do not understand.

Thank you.

Answer 1

a. 1504.6 invnorm(.97,0,1)*800 this is on the ti84
b. 800/sqrt(n)<50 800/50