My guess is 9 hours, because since pipe a takes 12 hours and with pipe b it takes 9, it's 12-9=3
I think.
The probability to choose letter n is

. After drawing letter n there left 25 letters and the probability to choose the letter w is

. After drawing two letters n and w there left 24 letters and then the probability to choose letter b is

. After drawing three letters n, w and b there left 23 letters, so the probability to choose letter t is

.
Using the product rule for probabilities, you can obtain that

.
0.75 kilometers per minute.. & 3.75 kilometers in 5 mins ....... im 100%