. In how many ways can 20 identical

. In how many ways can 20 identical pencils be distributed among three girls so
that each gets at least 1 pencil?
【Solution】
The first girl can take from 1 to 18 pencils. If she takes 1, the second girl can take
from 1 to 18. If she takes 2, the second girl can take from 1 to 17, and so on. The
third girl simply takes whatever is left. Hence the total number of ways is
18+17+…+1=19 18
2× =171.
ANS: 171

ผลลัพธ์ (อังกฤษ) 3:

.In how many ways can 20 be identical pencils distributed among three girls
so that each gets at least 1 pencil?

【 Solution 】 The first girl can take from 1 to 18 pencils. If she takes 1, the second girl
can take from 1 to 18.If she takes 2, the second girl can take from 1 to 17, and so on. The
third girl simply takes whatever is left.The total number of ways is Hence
1 = 19 18 17 18
...2 × = 171.
ANS: 171