A quick mental shortcut is to double the 5% interest to get 10%, since this occurs over 2 years. Then taking 10% of something is the same as moving the decimal point one spot to the left. So we go from 200 to 20
Another 10% increased means it becomes 110*110/100 = 121. The first 10% increase means the second 10% increase will be an actual 11% increase from the original, so it will be a total of 21%.
