Question

mrs. blat buys 1 or 2 books a month for $13.99 each. she has a magazine subscription that costs her $10.00 a month. the sunday paper is delivered each month for $6.00 a month. what is the total of her reading material for a year that is fixed?

Answer 1

Mrs. Blat spends at least ...
  13.99 +10.00 +6.00 = 29.99
each month. Over the year, this is a minimum of
  12*$29.99 = $359.88

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The variable cost is the cost of the 2nd book, which may or may not be purchased in any given month.