Have been seeing some problems like ... Initial volume of LakeX is V litres. Everyday there is 10% evaporation and 10% withdrawal from the previous day.

What's the volume left on the 20th day ?

How do you approach these kinda questions ?

If you have a particular problem in mind, please post it (assuming it's not from a banned source). The devil is in the details - we actually need to see the problem and the answer choices. My preferred method for something like the 20th day (!!) is to estimate but I can't tell for sure if that approach will work unless I see the answers.