Speed of a boat in still water is 18 km/h while the speed of stream is ____ km/h. Total time taken by the boat to go 90 km upstream and 210 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:
I. 7, 20
II. 5, 15
III. 9, 22.5
IV. 3, 16

For I: Speed of boat = 18 km/h Speed of stream = 7 km/h Speed in upstream = 18 – 7 = 11 km/h Speed in downstream = 18 + 7 = 25 km/h Total time = 90/11 + 210/25 = 8.18 + 8.4 = 16.58 hours So, I can’t be the answer. For II: Speed of boat = 18 km/h Speed of stream = 5 km/h Speed in upstream = 18 – 5 = 13 km/h Speed in downstream = 18 + 5 = 23 km/h Total time = 90/13 + 210/23 = 6.92 + 9.13 = 16.05 hours So, II can’t be the answer. For III: Speed of boat = 18 km/h Speed of stream = 9 km/h Speed in upstream = 18 – 9 = 9 km/h Speed in downstream = 18 + 9 = 27 km/h Total time = 90/9 + 210/27 = 10 + 7.78 = 17.78 hours So, III can’t be the answer. For IV:

x =3  → 90/15 + 210/21 = 6 + 10 = 16 So, IV can be the answer.