a total of 4500 people in the city bought take out pizza at least once during the week this was 20% of all the people who live in the city how many people live in the city

We can represent the total number of people who live in the city with the variable 'p'

20% is 0.2 in decimal form, so 20% of the people who live in the city can be represented as 0.2p.

Since 20% of people in city bought pizza, and 4500 people bought pizza, we can form an equation:

0.2p = 4500

Multiply both sides by 10 in order to get rid of the decimal (It's not necessary, but sure does make the calculation less confusing!):

0.2p * 10 = 4500 * 10

2p = 45000

p = 22500

22,500 people lived in the city.