Large fresh water reservoir is fitted with two types of feeder pipes- hot water pipes and cold water pipes. Six cold water pipes alone can fill the reservoir in 12 hours. 3 cold water pipes and 9 hot water pipes together can fill the reservoir in 8 hours. How long will 5 hot water pipes alone take to fill the reservoir? `18\ h r s\ 36\ m in` b. `\ 20\ h r s\ 45\ m in` c. `21\ h r s\ 36\ m in` d. none of these