FOUR (a) The lifespan of a certain make of electric light bulb is known to be normally distributed with a mean life of 2000 hours and a standard deviation of 120 hours. Find the probability that the lifespan of such bulbs will be i) greater than 2150 hours ii) greater than 1910 hours iii) less than 2500 hours iv) at least 3000 hours v) within the range of 1850 hours to 2090 hours