Now I see it. Twenty items (R operations) in 40 slots: 40!/(20!) and the order of the items does not matter, so again divide by 20!. The remaining slots are D o

Now I see it. Twenty items (R operations) in 40 slots: 40!/(20!) and the order of the items does not matter, so again divide by 20!. The remaining slots are D operations. The answer actually came to me in the shower this morning.