With all things equal, except weight and velocity, weight is equal to the square of velocity. Then the square 21 minus the square root of 20 divided by the square root of 20 times 100 gives the percentage of velocity increase, which appears to be 2.4695. The landing speed in this case should be 1.024695 times 40 or 40.9878. Now 41 is close enough to this. The key thing here is the square root of the weight determines the velocity.