Hi Candidate,

The time needed to throw the stone upwards and downwards in the entire journey is equal to 9 seconds. For upwards, V1 =u+10(t1), So V1= 10t1, and for Downwards journey- V2= V1 +10t2.

Now if we put the value of V1 with V2 we get- V2= u+10t1+10t2, where the value of t1+t2= 9 seconds, and u (initial)= 0. So the value of V2= 10*9=90 metres per second.

Hope that this answer helps you. Have a Good Day!!