Let X be the lifetime of an electronic device. It is known that the average lifetime of the device is 717 days and the standard deviation is 90 days. Let xbar be the sample mean of the lifetimes of 169 devices. The distribution of X is unknown, however, the distribution of xbar should be approximately normal according to the Central Limit Theorem. Calculate the following probabilities using the normal approximation.

(a) P(xbar≤707)=

Mainly, I've been playing around with pnorm with no success. Any suggestions are deeply appreciated.