# Thread: How to create a formula

1. ## How to create a formula

If a party cost £100 to hire a venue, and £10 per person for refreshments, five guests don't pay anything, but all other people will be asked to pay equal amounts, how do I know if the formula I create is in the right order?

I thought;

n = £100 + £10p

This gives me two unknowns and I am not sure how to work it out?

Any help much appreciated

2. ## Re: How to create a formula

your formula gives the total cost of the party. If you wants to calculate amount each person had to pay you divide it by p-5

3. ## Re: How to create a formula

Originally Posted by waqarhaider
your formula gives the total cost of the party. If you wants to calculate amount each person had to pay you divide it by p-5
Not sure how that works, I have not discovered how to find out how many people would attend?

4. ## Re: How to create a formula

The cost per person is the cost of the party [x(n)] divided by the number of people attending (n).

The cost of the party you (correctly) have as x(n) = £100-10n

The amount of people who pay are the n people who attend minus the 5 who do not pay (assuming they all attend)

$\displaystyle y(n) = \dfrac{100+10n}{n-5}$ where y(n) is the cost per person.

The value of x(n) or y(n) cannot be determined without a value of n.

5. ## Re: How to create a formula

Originally Posted by e^(i*pi)
$\displaystyle y(n) = \dfrac{100-10n}{n-5}$ where y(n) is the cost per person.
Typo. (100 + 10n) / (n - 5)

So if 8 people attend:
(100 + 80) / 3 = 180 / 3 = 60