In these questions, there is no hard and fast rule about which direction is positive and which is negative. You can set it yourself, depending on the situation, to make the question easier to answer. If something is being dropped, down is the obvious positive. If it is being thrown upwards, then up is the obvious positive.
In this case, the question says that the object is falling down. Take down as positive. So u = 5 m s-1, and as you say, a = 9.8 m s-2 (positive). s= 20 m. We can therefore work out v.
I don't make the answer 25 m s-1, however, unless you meant to say that it falls a further 30 m rather than 20 m.