# Set theory [Solved!]

**Mahamaya** 02 Jan 2016, 07:19

### My question

Can anyone pls help me with the solution to this brain bugging problem.

using venn diagram

In a school of 105 teachers, 51 teach mathematics and physics but none teach both math and chemistry. how many teach only physics?

### Relevant page

Counting and Probability - Introduction

### What I've done so far

Drawn many Venn diagrams, but I can't get it. It's driving me craaaazy!

X

Can anyone pls help me with the solution to this brain bugging problem.
using venn diagram
In a school of 105 teachers, 51 teach mathematics and physics but none teach both math and chemistry. how many teach only physics?

Relevant page
<a href="/counting-probability/counting-probability-intro.php">Counting and Probability - Introduction</a>
What I've done so far
Drawn many Venn diagrams, but I can't get it. It's driving me craaaazy!

## Re: Set theory

**Murray** 03 Jan 2016, 04:26

What is the size of your Universal set?

What is the formula for the intersection size of 2 sets?

X

What is the size of your Universal set?
What is the formula for the intersection size of 2 sets?

## Re: Set theory

**Mahamaya** 03 Jan 2016, 17:43

The universal set is 105.

Do you mean this formula?

`P(E_1 \cup E_2) = P(E_1) + P(E_2) - P(E_1 \cap E_2)`

X

The universal set is 105.
Do you mean this formula?
`P(E_1 \cup E_2) = P(E_1) + P(E_2) - P(E_1 \cap E_2)`

## Re: Set theory

**Murray** 04 Jan 2016, 20:36

Well, that's the formula for the probability of non-mutually exclusive events.

We need: `N(E_1 \cup E_2) = N(E_1) + N(E_2) - N(E_1 \cap E_2)`

X

Well, that's the formula for the probability of non-mutually exclusive events.
We need: `N(E_1 \cup E_2) = N(E_1) + N(E_2) - N(E_1 \cap E_2)`

## Re: Set theory

**Murray** 05 Jan 2016, 14:23

I've had closer look at your question and I believe there is not enough information. We would need to know some of the other intersections before we could solve this.

X

I've had closer look at your question and I believe there is not enough information. We would need to know some of the other intersections before we could solve this.

## Re: Set theory

**Mahamaya** 05 Jan 2016, 23:57

We complained and my teacher "withdrew" the question. I think she couldn't do it, either!

X

We complained and my teacher "withdrew" the question. I think she couldn't do it, either!

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