I studied for four hours. I added 2 more hours every day for a week. How many more days do I have to study until I have spend 20 hours studying?

Answer 1

you have an arithmetic series:

4 + 6 + 8 + ...
Easy to see that after 3 days, you will have already spent 18 hours,
so you need one extra day of only 2 more hours to have a total of 20 hrs.