Solution

Speed of boat C in still water = 15 km/hr Speed of stream = 3 km/hr Upstream speed = 12 km/hr Downstream speed = 18 km/hr Let, total time taken by boat C for the entire journey = 19t hours So, according to the question Time taken to travel upstream = 10t hours Time taken to travel downstream = 9t hours Let, a kms and b kms be the distance travelled by boat C upstream and downstream respectively. = 10t hours …(1) hours …(2) From equation 1 and 2, we get, a:b = 20:27 Total distance travelled by boat C = 141 kms So, upstream distance = 60 kms, and downstream distance = 81 kms Upstream time = = 5 hours Downstream time = = 4.5 hours Total time = 5 + 4.5 = 9.5 hours Hence, option 4 is the correct answer.