A stone is dropped from a tower of 100 meter height and another stone is thrown vertically up words with a velocity of 25 meter/second. When and where the two stones meet?

We're not going to do your homework for you. Please show us what efforts you have made to solve this problem, and where you're getting stuck - I'm sure someone will help you once you have demonstrated some effort.

a = 9.8 ms^-2
u = 0
s = 100 m

u = 25 m/s
a = - 9.8 ms^-2
v = 0 ms^-2

hmmm, interesting. I'll need a second look at this one

s = ut + 0.5 at^2
s = 25 t - 4.9 t^2 (this is your equation for stone 2)

s = 4.9t^2 (this is the equation for stone 1)

Yes equate these equations and I get

31.8877551 = s

t = 2.551020408

Not quite. Your equation for stone 1 ignores the fact that it is dropped from a height of 100m, and the sign of the acceleration due to gravity should be negative. This equation should be:

s = 100 - 4.9t^2.

Your equation for stone 2 is correct: s = 25t - 4.9t^2

Now, solve for t, and you'll get the correct answer.