1. Probability help

Hi, I'm stuck with this, if anyone can help, it'll be awesome!

1. Four-fifths of all volunteers working in a charity shop are over 50. If two volunteers are chosen at random, what is the probability that one of them is over 50, but the other one is not?

• 0.04
• 0.2
• 0.16
• 0.32
• None of the above
2. A producer of 'home-made' biscuits is known to produce biscuits whose thicknesses are Normally-distributed. If the mean thickness is 4mm with a standard deviation of 0.6mm, approximately what proportion of biscuits will be thicker than 5mm?

• 0.9525
• 0.0446
• 0.0475
• 0.4325
• None of the above

what is the quickest way or formula or working it out?

2. Originally Posted by chungkayan
Hi, I'm stuck with this, if anyone can help, it'll be awesome!

1. Four-fifths of all volunteers working in a charity shop are over 50. If two volunteers are chosen at random, what is the probability that one of them is over 50, but the other one is not?

• 0.04
• 0.2
• 0.16
• 0.32
• None of the above
2. A producer of 'home-made' biscuits is known to produce biscuits whose thicknesses are Normally-distributed. If the mean thickness is 4mm with a standard deviation of 0.6mm, approximately what proportion of biscuits will be thicker than 5mm?

• 0.9525
• 0.0446
• 0.0475
• 0.4325
• None of the above
what is the quickest way or formula or working it out?
1. Assuming a large population (so that it's essentially a sampling with replacement problem): X ~ Binomial(n = 2, p = 4/5). Calculate Pr(X = 1).

2. Pr(X > 50) = Pr(Z > (5 - 4)/0.6) = Pr(Z > 5/3) = 1 - Pr(Z < 5/3).

3. Sorry, call me slow. but i didn't really catch that. whats the formula for first one?

4. Originally Posted by chungkayan
Sorry, call me slow. but i didn't really catch that. whats the formula for first one?
Have you studied the binomial distribution?

5. No, I haven't is there a link to study it? many thanks

6. Originally Posted by chungkayan
No, I haven't is there a link to study it? many thanks
Binomial distribution - Wikipedia, the free encyclopedia