i have no idea on this one

The batteries from a certain manufacturer have a mean lifetime of 840 hours, with a standard deviation of 70 hours. Assuming that the lifetimes are normally distributed, complete the following statements. (a) Approximately 99.7% of the batteries have lifetimes between |:| hours and |:| hours . (b) Approximately ? v of the batteries have lifetimes between 770 hours and 910 hours