| A man goes downstream with a boat to some destination and returns upstream to his original place in 5 h. If the speed of the boat in still water and the stream are 10 km/h and 4 km/h respectively, then the distance of the destination from the starting place is [SSC (CGL) 2009] |
A) 16 km
B) 18 km
C) 21 km
D) 25 km
Correct Answer: C
Solution :
| Let the distance of the destination from the starting point = x km |
| Then, the rate downstream\[=(10+4)\,\,km/h=14km/h\]and rate upstream \[=(10-4)\,\,km/h=6km/h\] |
| According to the question, |
| \[\frac{x}{14}+\frac{x}{6}=5\] |
| \[\Rightarrow \] \[\frac{3x+7x}{42}=5\] |
| \[\Rightarrow \] \[10x=42\times 5\] |
| \[\Rightarrow \] \[x=\frac{42\times 5}{10}=21\] |
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