Time, Work, Speed

Which of the following statement is of inverse variation?

Area of circle and its radius.

Which of the following statement is of inverse variation?

Number of workers on a job and time taken by them to complete the job.

Which of the following statement is of inverse variation?

Number of pipes of same size to fill a tank and the time taken by them to fill the tank.

Which of the following statement is of inverse variation?

Petrol filled in the tank of a vehical and its cost

120 bags of half litre milk can be filled by a machine within 3 minutes find the time to fill such 1800 bags ?

A car with speed 60 km/hr  takes 8 hours to travel some distance. What should be the increase in the speed if the same distance is to be covered in 7\[\frac{1}{2}\] hours?

If 15 workers can build a wall in 48 hours, how many workers will be required to do the same work in 30 hours ?