Question

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

Answer 1

20 !/(20-r) !=13*(20 !/(20-(r-1)) !)

this equation is formed after breaking down the question

now in numerator 20 ! will cancel out from both sides:

1/(20-r) ! = 13 * (1/(20-r+1) !)

Now,we are just opening and solving the equation :

1/(20-r) ! = 13 * (1/(21-r) !)............................eq(1)

Now,we want (20-r) ! on both sides to cancel them out.

So (21-r)!=(21-r)(20-r)!

Now putting this value in eq(1)

1/(20-r) ! = 13 * (1/(21-r)(20-r)!)

the (20-r)! part will cancel out and we are left with only

1/1=13/(21-r)

solving it we get r = 7

Answer 2

Step-by-step explanation:

The Above Pic is the correct answer