Velocity and displacement *sigh* Isimple but seems comple

[Lexadis]

Homework Statement

A stone is thrown vertically up with a velocity of 20ms/s. The maximum height reached by the stone and the total time taken by it to fall on the ground are

(1) 20m and 2 s. (2) 20m and 4 s. (3) 40m and 2 s. (4) 40m and 4 s.

Homework Equations

The Attempt at a Solution

V = s/t
V = 20/1 (since the time taken for the stone is 2s to go up and then come falling down, I divided it by 2 and I chose the first answer as it seemed to get my favorable answer)
V = 20m/s.

But according to the scheme it's the second answer and I really don't understand how it can come, providing that even if they are divided you get the velocity 5m/s.

 

[Panphobia]

S = So + Vo*t + (1/2)*a*t^2
V = Vo + a*t

These are the equations that you need to use where

So = initial distance from origin
Vo = initial velocity
a = acceleration

Thats all you need to know to solve this question