X varies inversely as y, when x = 15 then y = 10, if x = 20 then y =?

Write the following statements using the symbol of variation.

(1) The wavelength of sound (l) and its frequency (f) are in inverse variation.

(2) The intensity (I) of light varies inversely with the square of the distance (d) of a screen from the lamp.

and when x = 40 then y = 16. If x = 10, find y.

Which of the following statements is of inverse variation?

(1) The number of workers on a job and time taken by them to complete the job.

(2) The number of pipes of the same size to fill a tank and the time taken by them to fill the tank.

(3) Petrol filled in the tank of a vehicle and its cost.

(4) Area of the circle and its radius.

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