Answer:
The answer is proportions.
Explanation:
To work backwards to find realized income needed from a given mortgage payment amount, you use proportions.
For example:
Say you are paying $1250 as monthly rent.
So, if you spend 25% of your monthly income on rent, you need a monthly income of $5000. If you can spend 30% in rent, then you will need a monthly income of $4166.66.
This way proportion is used to find the required answer.