1. If Nishu can swim downstream at 6kmph and upstream at 2kmph.What is his speed in still water ?
5 km/hr 4 km/hr 8 km/hr 7 km/hr
If the speed downloadstream is a km/ hr and the speed upstream is b km/ hr then Speed in still water is = ½ (a+b) km / hr [memory tool last 2 L cross and make +] Explanation: Given : speed downstream a = 6 km ph Speed upstream b = 2kmph Speed in still water = ½ (a+b) kmph = ½ (6+2) = 8/2 = 4kmph speed in still water = 4kmph
2.Ashok can row upstream at 8kmph and downstream at 12kmph.What is the speed of the stream ?
6 km/hr 3 km/hr 2 km/hr 4 km/hr
Basic Formula: If the speed downstream is a kmph and the speed upstream is b kmph then Speed of the stream = ½ (a-b) kmph Explanation: Speed downstream a = 12kmph Speed upstream b = 8 kmph Speed of the stream = ½ (a-b) = ½ (12-8) = 4/2 = 2 kmph speed of the stream = 2kmph
3.A man rows 750m in 775 seconds against the stream and returns in 7 1/2 minutes. What is rowing speed in still water ?
4.7km/hr 4 km/hr 3.5km/hr 6 km/hr
Basic Formula: i) Speed in still water = ½ (a+b) kmph where ‘a’ is speed downstream and ‘b’ is speed upstream ii) a km / hr = a x 5/18 m /s iii) a m/sec = a x 18/5 km/hr Explanation: Speed upstream ‘b’ = 750m / 775 sec = 30/31 m/sec Speed downstream ‘a’ = 750 m/ (15/2)minutes [ 1min=60 sec] a = 750m/450 sec =5/3 m/sec speed in still water = ½ (a+b) = ½ (750/450 + 750/675 ) m /sec = ½ (750/450 + 750/675 ) x 18/5 km/hr = ½ (5/3 + 30/31) x 18/5 km/hr = 4.7 km/hr
4.A man can row 9 (1/3) kmph in still water and finds that it takes him thrice as much time to row up than as to row down the same distance in the river. What is speed of the current ?
5km/hr 3(1/2) km/hr 4 (2/3) km/hr 8 (3/2)km/hr
Basic Formula: Speed of current = ½ (a-b) km/hr Explanation: Let man’s rate upstream be x km/hr. Then his rate downstream is 3 x km/hr Given: Speed in still water = 9 (1/3) = 28/3 km/hr i.e, ½ (a+b) = 28/3 km/hr ½ (x+3x) = 28/3 2x = 28/3 x = 28/ 2 x 3 = 14/3 km/hr rate upstream b = 14/3 km/hr and rate downstream a = 14/3 x 3 = 14 km/hr speed of the current = ½ (a-b) = ½ (14 – 14/3) = ½ (42-14/3) = 28/6 = 4 (2/3) km/hr
5.Sham can row a boat at 10kmph in still water. IF the speed of the stream is 6kmph, the time taken to row a distance of 80km down the stream is
4 hours 5 hours 3 hours 2 hours
Basic Formula: Speed of stream = ½ (a-b) km/hr Speed in still water = ½ (a+b) km/hr Explanation: Given: Speed in still water, ½ (a+b) = 10 km/hr a+b = 20 km/hr…………….(1) speed of the stream, ½ (a-b) = 6km/hr a-b = 12 km/hr …………….(2) (1)+(2 ) we get 2a = 32 a = 16 km/hr speed downstream =distance traveled / time taken time taken = 80/16 = 5 hours
5 km/hr 4 km/hr 8 km/hr 7 km/hr
If the speed downloadstream is a km/ hr and the speed upstream is b km/ hr then Speed in still water is = ½ (a+b) km / hr [memory tool last 2 L cross and make +] Explanation: Given : speed downstream a = 6 km ph Speed upstream b = 2kmph Speed in still water = ½ (a+b) kmph = ½ (6+2) = 8/2 = 4kmph speed in still water = 4kmph
2.Ashok can row upstream at 8kmph and downstream at 12kmph.What is the speed of the stream ?
6 km/hr 3 km/hr 2 km/hr 4 km/hr
Basic Formula: If the speed downstream is a kmph and the speed upstream is b kmph then Speed of the stream = ½ (a-b) kmph Explanation: Speed downstream a = 12kmph Speed upstream b = 8 kmph Speed of the stream = ½ (a-b) = ½ (12-8) = 4/2 = 2 kmph speed of the stream = 2kmph
3.A man rows 750m in 775 seconds against the stream and returns in 7 1/2 minutes. What is rowing speed in still water ?
4.7km/hr 4 km/hr 3.5km/hr 6 km/hr
Basic Formula: i) Speed in still water = ½ (a+b) kmph where ‘a’ is speed downstream and ‘b’ is speed upstream ii) a km / hr = a x 5/18 m /s iii) a m/sec = a x 18/5 km/hr Explanation: Speed upstream ‘b’ = 750m / 775 sec = 30/31 m/sec Speed downstream ‘a’ = 750 m/ (15/2)minutes [ 1min=60 sec] a = 750m/450 sec =5/3 m/sec speed in still water = ½ (a+b) = ½ (750/450 + 750/675 ) m /sec = ½ (750/450 + 750/675 ) x 18/5 km/hr = ½ (5/3 + 30/31) x 18/5 km/hr = 4.7 km/hr
4.A man can row 9 (1/3) kmph in still water and finds that it takes him thrice as much time to row up than as to row down the same distance in the river. What is speed of the current ?
5km/hr 3(1/2) km/hr 4 (2/3) km/hr 8 (3/2)km/hr
Basic Formula: Speed of current = ½ (a-b) km/hr Explanation: Let man’s rate upstream be x km/hr. Then his rate downstream is 3 x km/hr Given: Speed in still water = 9 (1/3) = 28/3 km/hr i.e, ½ (a+b) = 28/3 km/hr ½ (x+3x) = 28/3 2x = 28/3 x = 28/ 2 x 3 = 14/3 km/hr rate upstream b = 14/3 km/hr and rate downstream a = 14/3 x 3 = 14 km/hr speed of the current = ½ (a-b) = ½ (14 – 14/3) = ½ (42-14/3) = 28/6 = 4 (2/3) km/hr
5.Sham can row a boat at 10kmph in still water. IF the speed of the stream is 6kmph, the time taken to row a distance of 80km down the stream is
4 hours 5 hours 3 hours 2 hours
Basic Formula: Speed of stream = ½ (a-b) km/hr Speed in still water = ½ (a+b) km/hr Explanation: Given: Speed in still water, ½ (a+b) = 10 km/hr a+b = 20 km/hr…………….(1) speed of the stream, ½ (a-b) = 6km/hr a-b = 12 km/hr …………….(2) (1)+(2 ) we get 2a = 32 a = 16 km/hr speed downstream =distance traveled / time taken time taken = 80/16 = 5 hours
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