Question 1174007
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The other tutor showed a good typical formal algebraic solution.<br>
You can get a lot of valuable mental exercise by solving the problem informally, using logical reasoning and simple arithmetic.<br>
(1) $131 for 7 shirts makes the average cost about $18.70.  Since that is much closer to $15 than to $28, most of the shirts must be the less expensive plain shirts.<br>
(2) The cost of each fancy shirt is $28, so the cost of any number of fancy shirts is an even number.  Since the cost of each plain shirt is an odd number, and the total cost of all the shirts is an odd number, the number of plain shirts must be odd.<br>
Either one of those alone, or both together, would lead to the guess that she bought 5 plain shirts and 2 fancy shirts.  And trying that combination shows it is the right one: 5(15)+2(28) = 75+56 = 131.<br>