One mistake that people often make is assuming that there are 4 weeks in a month when they want to calculate expenses. The issues is: then you would only be calculating 48 weeks in a year (4 weeks/month x 12 months/year = 48 weeks/year) instead of 52. To avoid this mistake, you should work to convert weeks into 1 year, and then 1 year into a month. If you get paid $50 a week, how much do you make each month?