It's even not only about their math skill, it's also about self control. They just need to wait for a while and put small amount of money into an account each w

It's even not only about their math skill, it's also about self control. They just need to wait for a while and put small amount of money into an account each week. They could get a TV/couch/iPad within <72 weeks. Considering their financial status, why they want to buy those things right NOW?