a) At heighest poing velocity will be 0

so by formula v² = u² - 2gh        (here - is because you are moving against gravity)

so h = 20*20/2*10 = 20 m

so ball will rise upto 25+20 m from the ground.

b) in second part we have to calculate the time of journey from top of 25 m high building to ground.

for this we should use  

-h = ut - gt²/2                   [in this case we are using upward direction as + ve and downward as - ve]

so -25 = 20t - 10t²/2

or 5t² - 20t - 25 = 0

or t²- 4t - 5 = 0

or (t+1)(t-5) = 0  

or t = 5 , -1

so t = 5 sec.