Plan: Credit card debt, mortgages, saving for retirement.

Started with a discussion of the I131 decay problem from the text – it asks to find both the total I131 content of a 3.5 kg fish and the time until the fish is safe, given safe level of 2000 becquerels per kg, initial level 10000 becquerels per kg and a half life of 8 days. It’s a bad question. The total content is an easy units computation, but not useful. The only interesting valuable question is the one asking for time to safety – everything there is per kg.

We did what I’d planned. As usual, about a third of the students had credit cards. The most interesting part of the discussion – not particularly quantitative – was talking about the credit card company’s source of income – merchant payments, not just interest payments from customers. We spent more time on credit scores than in the past – and the paradox that you need a history of paying off debts to have a good credit score, so never having had a credit card can be a disadvantage.

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