A stone is thrown vertically upwards and the height reached by it in time t is given by  $S=40t-4t^2$  then the stone reaches the maximum height is

$S=40t-4t^2$

          V = 40 – 8t= 0

          $t=\frac{40}{8}=5\sec$

          Max height = 40(5) – 4 (25)

                             = 200 – 100

                             = 100