Time from tower to top, t = $\frac{u-v}{g}$
= $\frac{20-0}{10}$
= 2s
distance from tower to top
x = ut - $\frac{1}{2}$gt²
= (20 x 2) - ($\frac{1}{2}$ x 10 x 2 x 2)
= 20m
Time from the top to the ground, t = 6 - 2
= 4s
distance from top to ground,
x = ut - $\frac{1}{2}$gt²
= (0 x 4) + ($\frac{1}{2}$ x 10 x 4 x 4)
= 80cm
therefore, height of tower = 80 - 20
= 60m