`(20P_(r-1))/a=(20P_r)/1=(20P_(r+1))/6` then `a`

(20P_(r-1))/(a)=(20P_(r))/(1)=(20_(r+1))/(6) then a=

(20P_(r-1))/(a)=(20P_(r))/(3)=(20P_(r+1))/(4) then a=

if 20C_(r-1)=20C_(r+1) then rC_(5)

IF ""^(20)P_(r):""^(20)P_(r-1)=15 :1 then r=

If 20C_(r+1)=^20C_(r-1) , then r is equal to:

If ""^(20)P_(r)=13xx""^(20)P_(r-1) find r.

Find 'r' if : (i) ""^(5)P_(r )= ""^(6)P_(r-1) (ii) 5""^(4)P_(r )=6""^(5)P_(r-1) (iii) ""^(5)P_(r )=2""^(6)P_(r-1) (iv) ""^(10)P_(r+1): ""^(11)P_(r )=30 :11 .

Find r if (i) .^(5)P_(r) = 2 .^(6)P_(r-1) (ii) .^(5)P_(r) =^(6)P_(r-1) .