Question

A ball is throwing downward with an initial velocity of 10 m/sec. If the final velocity is 20 m/s, what is the time?

Answer 1

The equation you want is v = u + at, where u = initial velocity; v = final velocity; a = acceleration and t = time. v = 20. u = 10. a = 9.81 m/sec, though I'd guess from the numbers supplied that you may be expected to assume that a is approximately 10. You'll then be able to work it out from the above.