Originally Posted by

**arze** Letters are sent by post and the probability that they are delivered in days are:

days: 1 2 3

1st class: 0.9 0.1 0

2nd class: 0.1 0.6 0.3

A secretary sends 4 letters and the receivers reply to them immediately on the same class mail. find the probability of receiving at least two replies within three days using second class post?

P(reply in 3 days)=$\displaystyle 0.1\times 0.1+2\times 0/1\times 0.6=0.13$ <= did you mean 2*0.1*0.6?

P(1 reply in 3 days)=$\displaystyle 0.13\times (0.87)^{3}=0.0856$

P(no replies in 3 days)=$\displaystyle 0.87^4=0.573$

P(at least 2 replies)=$\displaystyle 1-(0.0856+0.573)=0.341$

Answer = 0.085

What have i done wrong? I have spent quite some time over it and can't see where i have missed anything

Thanks