You mean of it happening within 20 days?
Well if there is a 3/10 probability of it happening in any given day, there is a 7/10 probability of it not happening on any given day. The probability of it not happening at all in 20 consecutive days is:
(7/10)^20
So the probability of it happening at least once in 20 days is:
1-(7/10)^20≈9992/10000 (about 99.92%)
