In a class when a test is conducted, mean of 25 students marks is 60 if mean of first 13 students marks is 70 and mean of last 13 students marks is 50 find the marks of the middle student when arranged in the ascending order.
A man, rowing a boat, covers a distance of 16 km in two hours downstream. He covers half the distance in same amount of time when rowing upstream. Calculate the speed of the stream.