If `^0P_5+5^9P_4=^(10)P_r ,` find the value of `rdot`

Explanation

`.^(10)P_(r )=.^(9)P_(5)+5^(9)P_(4)`
`=(9!)/((9-5)!)+5xx(9!)/((9-4)!)`
`=(9!)/(4!)+(9!)/(4!)`
`=2xx(9!)/(4!)`
`=(5xx2xx9!)/(5xx4!)`
`=(10xx9!)/(5!)`
`=(10!)/(5!)`
`=.^(10)P_(5)`
`implies r=5`