Suppose a brand of light bulbs is normally distributed, with a mean life of 1800 hr and a standard deviation of 100 hr. Find the probability that a light bulb of that brand lasts between 1690 hr and 1980 hr. The probability that a light bulb will last between 1690hr and 1980 hr is?

I need help with the methods of solving simplified for someone who does not use these equations on a daily basis. I can not understand the text book, is there an easier method to retain?