Question

Find r,if 15c4+15c5+16c6+17c7=18cr

Answer 1

Given :

15C4 + 15C5 + 16C6 + 17C7 = 18Cr ....(1)

1. Using property of combination

i.e nCr + nC(r-1) = (n+1)Cr

15C4 + 15C5 = 16C5

Substituting this in (1)

16C5 + 16C6 + 17C7 = 18Cr ....(2)

2. Again using property

16C5 + 16C6 = 17C6

Substitute this in (2)

17C6 + 17C7 = 18Cr .....(3)

3. Again using property

17C6 + 17C7 = 18C7

Substitute this in (3)

18C7 = 18Cr

4. From this we get r = 7

Answer 2

Answer:

The step by step explanation