The force, F, of the wind blowing on a vertical surface varies jointly as the area, A, of the surface and the square of the velocity, v. If a wind of 20 miles per hour exerts a force of 50 lb on a surface of 1/2
ft2, solve for the force a wind of 30 miles per hour will exert on a surface of 5 ft2.

You may use k to represent the constant of variation in your working if necessary.