The lifetime of lightbulbs are normally distributed with Mean = 5,197 hours and SD = 257 hours, If we want to be sure that 98% of all bulbs last longer than the advertised figure, what figure should be advertised?

(Tips This is to ask you to find a particular x-value so that P(X>x-value) = 0.9800 or very close;

This can be converted as: P(Z > (x-value - )/) ) = 0.9800

Now go to the normal table and look inside to find a value that is closest to 0.9800 and find the z-value: you should be able to find z-value = 2.05.

Now, you have (x-value - )/ = 2.05. And you know and , X-value = - z-value * )