A work-force of 420 men with a contractor can finish a certain piece of work in 9 months. How many extra men must he employ to complete the job in 7 months?

If 36 men can do a piece of work in 25 days, in how many days will 15 men do it?

A work force of 50 men with a contractor can finish a piece of work in 5 months. In how many months the same work can be completed by 125 men?

1200 men can finish a stock of food in 35 days. How many more men should join them so that the same stock may last for 25 days?

In a hostel of 50 girls, there are food provisions for 40 days. If 30 more girls join the hostel, how long will these provisions last?