# Probability question - it draw me mad

Discussion in 'Private tutors' started by Carlaxyz, Sep 30, 2020.

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1. ### CarlaxyzNew commenter

HI,
I need your help with one probability question, I am refreshing my maths to comeback to teaching and today is probability day, I have one question which I am not able to do and I am not sure why. Question is from old book by Brian Speed - higher mathematics for GCSE-

the full question :

Ann has a bargain box of tins. they are unlabelled but she knows that 6 tins contain soup and 4 contain peaches.

a) she opens 2 tins. what is probability that
(i) they are both soup, (ii) they are both peaches?

b) what is the probability that she has to open 2 tins before she gets a tin of peaches?
c) what is the probability that she has to open 3 tins before she gets a tin of peaches?
d) what is the probability that she get a tin of soup if she open 5 tins?

question a is fine but b, c, d, are a bit of a problem. I do not give you the answers as I want to see what would be your solutions (maybe the solutions in book are wrong, ) also I am interested why I cannot do one question when I did today something like 100 full exam questions (most of them tricky one) without any problem where this one I cannot even crack it with answers (they in form final solution)
thank you for any help
Carla

2. ### gainlyStar commenter

b) 6/10x4/9 (1st tin soup, 2nd peaches)
c) 6/10x5/9x4/8 (1st and 2nd soup, 3rd peaches)
d) 1 (if she opens 5 tins at least one must be soup)

3. ### shakes1616Established commenter

Draw a tree diagram and work it out that way.

4. ### CarlaxyzNew commenter

thanks, English is not my first language and even after 15 years living in uk it looks that I still have a trouble with some little things, so I understood that (b) two tins must be opened to have a next one tin with peaches. It looks that understanding a question is the most crucial part of solving a problems.
it really draw me crazy