5. Study the pictures given. On the pairs of given pictures answer the questions related to this. The speed of boat in still water is 11 km/h. It can go 12 km upstream and return downstream to the original point in 2 hours 45 minutes. Assume that speed of stream is ?km/h. (i) Total speed during upstream is (a) (11+? km/h (b) ?km/h (c) 11 km/h (d) (11−? km/h (ii) Total speed during downstream is (a) (11+? km/h (b) ?km/h (c) 11 km/h (d) (11−? km/h (iii) Speed of steam = (a) 5 m/s (b) 15 m (c) 5 km/h (d) 5 km (iv) Time taken during upstream (a) 45 min (b) 2 min (c) 2 hr. (d) 2 hr. 45 min (v) Time taken during downstream (a) 45 min (b) 2 min (c) 2 hr. (d) 2 hr. 45 min
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Let the speed of the stream be skm/hr.
Speed of boat in still water =11km/hr
Speed of boat in upstream =(11−s)km/hr
Speed of boat in downstream =(11+s)km/hr
Distance =12km
Therefore
11−s
12
+
11+s
12
=2
4
3
hr
⇒
(11−s)(11+s)
12(11+s+11−s)
=
4
11
⇒
121−s
2
12×22
=
4
11
⇒
121−s
2
12×2
=
4
1
⇒96=121−s
2
⇒s
2
=25
⇒s=±5
Since speed cannot be negative, we take s=5
Therefore Speed of stream is 5km/hr
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