Answer:
Mean = 339 hours
Standard deviation = 54 hours
Step-by-step explanation:
We are given that approximately 2.5% of the bulbs have lives exceeding 445 hours
So,P(x>445) = 0.025
P(x<445) =1- 0.025 =0.975
z value for 0.975 is 1.96
Formula :
So,
--A
Now we are given that approximately 16% have lives exceeding 393 hours,
So,P(x>393) = 0.16
P(x<393) =1- 0.16 =0.84
z value for 0.84 is 1
Formula :
So,
---B
Solve A and B
Substitute the value of from B in A
Substitute the value in B
So, mean = 339 hours
Standard deviation = 54 hours