The mean = 2,000 s = Sigma ( standard deviation ) = 300 M - 2 s = 2,000- 2 * 300 = 2,000 - 600 = 1,400 M + 2 s = 2,000 + 2 * 300 = 2,000 + 600 = 2,600 Answer. Useful life of light bulbs within 2 standard deviations of the mean is: A ) Between 1,400 and 2,600 hours