Devin is a kickboxing instructor who will be teaching classes at a local gym. To get certified as an instructor, he spent a total of $150. Devin will be earning a base salary of $100 per month from the gym, plus an additional $10 for every class he teaches. If Devin teaches a certain number of classes during his first month as an instructor, he will earn back the amount he spent on certification. How many classes will that take?