P and C

[nikhilagrawal]

My name is AJEET. But my son accidentally types the name by interchanging a pair of letters in my name. What is the probability that despite this interchange, the name remains unchanged?

a) 5%
b) 10%
c) 20%
d) 25%

[sudhir3127]

My answer is 10 %

2/20= 10%

[reachac]

10%

[nikhilagrawal]

can some one help me in determing the no of successful events.
I amy confused how we are getting 20 .. pls explain ..

[acecoolan]

Can someone explain this ....my Probability and Perm/Comb fundaes are totally 0 ..I think

Can anyone also suggest where to start afresh on these topics?

[Stuart@KaplanGMAT]

My name is AJEET. But my son accidentally types the name by interchanging a pair of letters in my name. What is the probability that despite this interchange, the name remains unchanged?

a) 5%
b) 10%
c) 20%
d) 25%

There are 5C2 different pairs of letters in your name.

5C2 = 5!/2!3! = 5*4/2 = 10

So, there are 10 swaps that your son could make.

Only 1 of those swaps (E/E) would leave your name spelled correctly. So, there's a 1/10 chance that your name will still be fine: choose (b).

Just curious, why aren't there 5 answer choices?