x Turn on thread page Beta
 You are Here: Home >< Maths

# Maths GCSE help watch

1. I'm stuck on this question: there are 7 red counters in a bag, the rest are blue. 2 counters were taken at random from the bag. The probability that here will be one of each colour is 7/15. What is the total number of colours in the bag?

I have tried to form a probability tree and I came to the (wrong) conclusion, n being number of counters, that ((7/n) x ((n-7)/(n-1))) x (((n-7/n) x (7/(n-1))) = 7/15 - sorry I realise there are an unreal amount of brackets in this but I tried to make it as clear as possible... that kinda backfired lol
If anyone could put me on the right track I would appreciate it! Or if you're feeling kind DM me the solution
2. (Original post by mundosinfin)
I'm stuck on this question: there are 7 red counters in a bag, the rest are blue. 2 counters were taken at random from the bag. The probability that here will be one of each colour is 7/15. What is the total number of colours in the bag?

I have tried to form a probability tree and I came to the (wrong) conclusion, n being number of counters, that ((7/n) x ((n-7)/(n-1))) x (((n-7/n) x (7/(n-1))) = 7/15 - sorry I realise there are an unreal amount of brackets in this but I tried to make it as clear as possible... that kinda backfired lol
If anyone could put me on the right track I would appreciate it! Or if you're feeling kind DM me the solution
Very close - just one small slip. Should be:

I trust you understand why.

I get two possible values for n:
Spoiler:
Show

10 or 21

3. (Original post by ghostwalker)
Very close - just one small slip. Should be:

I trust you understand why.

I get two possible values for n:
Spoiler:
Show

10 or 21

Ahhh that was careless haha, it’s good to see I was on the right track anyway. Thanks!

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 3, 2018
Today on TSR

### University rankings 2019

Cambridge at number one

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams