The number of ways to distribute 10 identical red pens and 14 identical blue pens among four persons such that each person gets 6 pens, is .
Answer
206
Solution
Once the red pens ri of each person are fixed, blue pens 6 - ri are fixed (they total 24 - 10 = 14 automatically).
Count r1 + r2 + r3 + r4 = 10, 0 ri 6.
Without the upper bound: 133 = 286.
Some ri 7: put ri = 7 + ri', remaining sum 3: 4 63 = 80 (two such ri impossible).
Required = 286 - 80 = 206
Answer: 206