| A boat takes 90 min less to travel 36 miles downstream than to travel the same distance upstream. If the speed of the boat in still water is 10 m/h, the speed of the stream is |
A) 2 m/h
B) 2.5 m/h
C) 3 m/h
D) 4 m/h
Correct Answer: A
Solution :
| Let the speed of the stream be x m/h. Then, |
| Speed downstream \[=(10+x)\,m/h\] |
| Speed upstream \[=(10-x)\,m/h,\] |
 \[\frac{36}{(10-x)}-\frac{36}{(10+x)}=\frac{90}{60}\] |
 \[72x\times 60=90\,(100-{{x}^{2}})\] |
| \[{{x}^{2}}+48x-100=0\] |
| \[(x+50)(x-2)=0\] |
| \[x=2\,m/h\] |
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