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Elementary school mathematics permutation and combination formula
Plan e doesn't add anything, just hamburgers with bread and meat.

One feed: **4 schemes.

Two kinds of feeding: **3+2+ 1=6 schemes.

Three kinds of feeding: **2+ 1+ 1=4 schemes.

Four kinds of feeding: 1 scheme.

To sum up, * * *1+4+6+4+1=16 is optional.