" 6.(i) If "^(20)P_(r)=6840," find "r

(i) If ""^(15)P_(r)=2730, find the value of r. (ii) If ""^(10)P_(r)=5040, find the value of r.

If ^20P_r=6840 , find r.

(i) If ""^(5)P_(r)=2xx""^(6)P_(r-1) , find r. (ii) If ""^(20)P_(r)=13xx""^(20)P_(r-1) , find r. (iii) If ""^(11)P_(r)=""^(12)P_(r-1) , find r.

If ^(22)P_(r+1) : ^(20)P_(r+2)=11 : 52 ,find r

(i) If .^(20)C_(r)=.^(20)C_(r+6) , find r. (ii) If .^(18)C_(r)=.^(18)C_(r+2) , find .^(r)C_(5) .

If P(5, r)=P(6, r-1) ,find r.

If ""^(22)P_(r+1):""^(20)P_(r+2)=11:52, find r.