Assertion :If p, q < r, the number of different selections of p+q things taking r at a time, where p things are identical and q other things are identical, is p+q−r+1. Reason: If p, q > r, the number of different selections of p+q things taking r at a time, where p things are identical and q other things are identical, is r-1.
Case number | Selection from p | Selection from q |
1 | p | r-p |
2 | p-1 | r-p+1 |
... | ... | ... |
p-r+q | r-q+1 | q-1 |
p-r+q+1 | r-q | q |
Case number | Selection from p | Selection from q |
1 | r | 0 |
2 | r-1 | 1 |
... | ... | ... |
r | 1 | r-1 |
r+1 | 0 | r |