Let n 1 < n 2 < n 3 < n 4 < n 5 be positive integers such that n 1 + n 2 + n 3 + n 4 + n 5 =…

Let n1<n2<n3<n4<n5 be positive integers such that n1+n2+n3+n4+n5=20 . Then the number of such distinct arrangements n1, n2, n3, n4, n5 is _________

Solution

When n5 takes value from 10 to 6 the carry forward moves from 0 to 4 which can be arranged in 4C0+4C14+4C22+4C41=7
Alternate solution
Possible solutions are
1, 2, 3, 4, 10
1, 2, 3, 5, 9
1, 2, 3, 6, 8
1, 2, 4, 5, 8
1, 2, 4, 6, 7
1, 3, 4, 5, 7
2, 3, 4, 5, 6
Hence 7 solutions are there.

Asked in: JEE Advanced 2014 (Paper 1)

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