Speed of a boat in still water is 10 km/h while the speed of stream is ____ km/h.
Total time taken by the boat to go 72 km upstream and 144 km downstream is ____ hours. The values given in which of the following options will fill the blanks in the same order in which it is given to make the above statement true:
A. 2, 21
B. 6, 27
C. 8, 44
D. 3, 30

For A: Speed of boat = 10 km/h
Speed of stream = 2 km/h
Speed in upstream = 10 – 2 = 8 km/h
Speed in downstream = 10 + 2 = 12 km/h
Total time = 72/8 + 144/12 = 9 + 12 = 21 hours
So, A can be the answer. For B: Speed of boat = 10 km/h
Speed of stream = 6 km/h
Speed in upstream = 10 – 6 = 4 km/h
Speed in downstream = 10 + 6 = 16 km/h
Total time = 72/4 + 144/16 = 18 + 9 = 27 hours
So, B can be the answer. For C: Speed of boat = 10 km/h
Speed of stream = 8 km/h
Speed in upstream = 10 – 8 = 2 km/h
Speed in downstream = 10 + 8 = 18 km/h
Total time = 72/2 + 144/18 = 36 + 8 = 44 hours
So, C can be the answer. For D: Speed of boat = 10 km/h
Speed of stream = 3 km/h
Speed in upstream = 10 – 3 = 7 km/h
Speed in downstream = 10 + 3 = 13 km/h
Total time = 72/7 + 144/13 ≈ 10.29 + 11.08 ≈ 21.37 hours (not 30)
So, D can’t be the answer. Hence, option d (Only A, B and C).