a body is thrown horizontally from tower 100 m high with a velocity 10 m/s. it is moving at an angle of 45 degree with horizontal after time in seconds.

Given,
h = 100m,
u = 10 m/s
Ɵ = 45^o
After time t(in s)

The horizontal component of velocity is constant as no horizontal acceleration is present.
so, 
u = v cos​Ɵ
where, v is velocity of projectile after some (required) time
so, v = u sec​Ɵ = (10m/s)(​√2) = 10​√2 m/s

Now,
we know,
u_y = initial velocity along y axis = 0
and,
v_y = v sin​Ɵ = (10​√2 m/s) (1/​√2) = 10 m/s

so, using v = u + at along y axis,
10m/s = 0 + (9.8m/s/s)t
so, t = 10/9.8 s = ​1.02040816 seconds ≈ 1.02 seconds