A salesman makes 50 calls during a particular week. You may assume that independently for each house visited, the probability of a sale is 0.2.
a] Find the probability that during this week, he makes
i) exactly 12 sales,
ii) between 10 and 14 (both inclusive) sales,
iii) his first sale on the third house visited. 
b] At the end of the week, he is paid £100 plus a commission of £50 for every sale. Find the mean and standard deviation of his total pay for this week. 
For a]i), I used the binomial formula and got 0.1033 (to 4dp)
For ii) I did P(X 10) - P(X 15) and taking these from my table values, I got 0.4956.
For iii) I got confused - but I think it would be done by failure x failure x success which is 0.8 x 0.8 x 0.2 which is 0.128. Can't say I'm too confident about this.
But what about part b? I've never known a question like this and hope it doesn't come up in the exam. How would you answer this?
Thanks if you can help