Have I Discovered Another Mathematical Football Coaching Error?

[VarlosZ]

This should help. So might this, for those who are into this sort of thing.

So, based on the table in the first link:

1) About 21-25%.

2) About 33-36%

Not based on the table, but I'd say that (3) is about 2-5%.

Note that the table is giving numbers for starting from your own 10 or 30 (not your 20), and with 2 minutes left (not 2:30), so my numbers are just guesses as to where the numbers in OP's scenarios would fall.