maths 問題,3條(f.5)

1.a man bought a certain number of rabbits for $1600. After one week, 5 rabbits died. The man then sold the remaining rabbits each at $40 more than the orginal cost per rabbit. He made a total profit of $200. Find the number of rabbits that the man bought.

2.A man rows againest the current for a distance of 12 km in a stream flowing at 4km/h. Then he turns round and rows back with the current to his starting point. If the whole trip takes 4 hours, find the man's rowing speed in still water.

3.martin can finish a job N days and john can finish the same job in (N+2) days. If they work together, they can finish the job in 1/6(N+6) days. Find the value of N.


要有step,完整的
thx~