Question

If in a big hole 1 drop falls for 1 st minute and 2 drops falls for 2 nd minute and it doubles for the next minute and the pond fulls after 20 minutes than when was the pond half.

Answer 1

On $20^{th}$ min = Full
Therefore, on $19^{th}$ min = Full/2
Full/2 = Half

Thus the pond was half on the $19^{th}$ minute