During your first week of training for a marathon, you run 10 total miles. You increase the distance you run by 20% each week. How many miles do you run during your 12th week of training?

Multiply 20% by ten to get 2. This will be the number of miles increased each week. Multiply 2 by 12 to get the total number of miles ran since the start of the increase. Add 10 miles and you will get 34 as your final answer.