Answer-A
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