1.

Find r if P (20,r)=13.P(20,r-1).

Answer»

SOLUTION :`""^20P_r=13xx""^20P_(R-1)`
`or,(20!)/((20!-r)!)=13xx(20!)/((20-r+1)!)`
`or,(20-r)! =((21-r)(20-r)!)/(13)`
`or,21-r=13`
`or,r=8`


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