Question

a ball thrown upward with a velocity of 100 m/s.It will reach the ground after?

Answer 1

v=u-gt
v=0 and u=100
0=100-gt
100=gt
t=100/9.8
t=10.2 
it takes 10.2 se to go upwards and same time reach ground then the total time s  10.2+10.2=20.4

Answer 2

v=u-gt 
v=0 and u=100 
0=100-g t 
100=gt 
t=100/9.8 
total time =10.2+10.2=20.4 sec.                      Answer.................