Question 870617
Find the z-score


z = (x-mu)/sigma
z = (1155-1200)/75
z = -0.60



Use <a href="http://www.math.upenn.edu/~chhays/zscoretable.pdf">this table</a> to find that the area to the left of z = -0.60 is 0.2743


So the area to the right of z = -0.60 is 1 - 0.2743 = 0.7257



Answer: the probability that a light bulb chosen at random will last more than 1155 hours is approximately <font color="red">0.7257</font> (about a 72.57% chance)