Another type of problem sums involve before-after scenarios, where the difference between two quantities remain unchanged. An example is shown in the picture.
Recognising the question is half the battle won. Typically the question tells you a ratio of the "before" scenario. After an equal amount is removed / added / after a number of years, the ratio is changed to something else.
To solve it, find the difference in the two ratios, and express both ratios such that the difference in the ratio before is the same as the difference in the ratio after.
More examples to come!