Originally Posted by

**fardeen_gen** Using first derivative test, since we need maximum height(say 'h') in time 't',

dh/dt = 0

d(20t - 5t^2)/dt = 0

20 - 5*2t = 0 [Using constant rule ; dt/dt = 1 ; power rule]

10t = 20

t = 2

Therefore, for t = 2 seconds, height is maximum

Putting t = 2 in equation for h,

h = (20*2 - 5 *(2)^2) m

= (40 - 5*4) m

= 20 m

For total distance, since stone reaches a maximum distance of 20 m above ground, total distance is (20 + 20) m, i.e. 40 m.(since stone reaches highest point, then falls back to the ground.)