(II) The luminous efficiency of a lightbulb is the ratio of luminous flux to electric power input. (a) What is the luminous efficiency of a $100-\mathrm{W}, 1700-\mathrm{lm}$ bulb? $(b)$ How many $40-\mathrm{W}, 60-\mathrm{Im} / \mathrm{W}$ fluorescent lamps would be needed to provide an illuminance of $250 \mathrm{Im} / \mathrm{m}^{2}$ on a factory floor of area $25 \mathrm{~m} \times 30 \mathrm{~m} ?$ Assume the lights are $10 \mathrm{~m}$ above the floor and that half their flux reaches the floor.