Let k be a positive integer. Suppose that the integers 1, 2, 3, ...3k, 3k + 1 are written down in random order.
What is the probability that at no time during this process, the sum of the integers that have been written up to that time is divisible by 3?
Source: Putnam competition
(In reply to
re: Solution by Charlie)
Comparing my computer results with the results of Brian's formula:
k computer formula
1 0.2500000 0.250000000
2 0.0714286 0.035714286
3 0.0200000 0.003333333
4 0.0054945 0.000228938
5 0.0014881 0.000012401
the computer result seems to be k! times that of the formula indicating the formula should be
k!*(k+1)!/((2k)!*(3k+1))
I'll look over Brian's derivation to see if this makes sense in that context.

Posted by Charlie
on 20160317 21:34:14 