conditional probability problem

**1. The problem statement, all variables and given/known data**

I've spent a few hours but unable to work out the solution of the following questions successfully.

The question is as follows:

http://www.4freeimagehost.com/uploads/2c43ac306dac.jpg

I am having problems with (a)(iii) and (b)(ii)

My attempts were as follows which were unsuccessful....

(a)(iii) Although I can work out the solution at last, my original attempt was incorrect but I don't understand why.

My first attempt:

P(pass) = P(4 answer correct) + P(5 answer correct) + P(all answer correct)

= (3x3x1x1x1x1)/(6x6x6x6x6x6) + (3x1x1x1x1x1)/(6x6x6x6x6x6) + (1x1x1x1x1x1)/(6x6x6x6x6x6)

However, this is wrong. But I don't understand why............

(b)(iii) I can't work out the solution at all. Please help. Thank you.

Re: conditional probability problem

If guessing probability of getting a question correct =1/4

So this is the binomial distribution B(6,1/4)