2026 · Q18
A person goes downstream at x km/h and upstream at y km/h by the same boat. What is the ratio of speed of boat in still water to speed of water?
Elementary MathematicsTime, Speed and Distance
- Ax/y
- By/x
- C(x + y)/(x - y)
- D(x - y)/(x + y)
Explanation
Let the boat's speed be b and the stream's speed be w: b+w=x (downstream) and b−w=y (upstream). Adding gives b=(x+y)/2 and subtracting gives w=(x−y)/2, so the required ratio b:w is (x+y):(x−y), option C. The common mistake is forming the ratio the wrong way round as (x−y):(x+y).