What should come in place of the question mark '?' in the following number series?

77, 76, 84, 57, ?, – 4

A woman invests Rs. 2000 at the start of each year at 5% compound interest per annum. How much will her investment be at the end of the 2^nd year?

A varies jointly with B and C. A = 6 when B = 3 and C = 2. Find A when B = 5 and C = 7.

If the height of a perpendicular cylinder is 10 cm and its curved surface area is 440 square cm, what is its radius?

In a mixture 25 litres, the ratio of milk and water is 4:1. How many litres of milk must be added to make the ratio 16:1?

What value will come in place of question mark (?) in the questions given below?

837 ÷ ? × 0.8 = 9.3 

What value will come in place of question mark (?) in the questions given below?

663.48 + 22.51 - 292.63 = ? + 300.44 

A man rows from J to K (upstream) and back from K to J (downstream) in a total time of 15 hours. The distance between J and K is 300 km. The time taken by the man to row 9 km downstream is identical to the time taken by him to row 3 km upstream. What is the approximate speed of the boat in still water?