Aptitude - Puzzle

Puzzle

1.A alone can do a work in 6 days B alone can do  in 8 days with help of c they finished the work
in 3 days.If the agreed sum is 640 what is the share of c.(refer r.s agarwal)

2.A drives a car four times a lap 10,20 30,60  kmph what is the average speed.

3.speed of boat in still water 10 km,if speed up  stream is 24 km and speed down stream is 16
what is speed of the river.

4.If grand father age is sum all the three  grand childern whos age r in equal interval what is the age of the grand father? (refer r.s agarwal)

5.In a grass field if 40 cow could eat for 40 days.The same grass field can feed 30 cows for60 days.how long will it feed 20 cows?

That what i remember.........