(i) Simple Rate Problems: The "Unitary Analysis" method will be used to solve many problems that involve proportions, without mentioning the word "proportion". (This is the method that I was taught in Grade 4 at my local P.S. 139 in Brooklyn, New York City, a half century ago when Dr. Dickler, was principal.) In fact, such problems can and should serve as background for formal proportional thinking.

Problem 1. Three T-shirts cost 15 dollars. How much do 5 T- shirts cost?

- http://www.math.umd.edu/~jnd/Difficult_Word_Problems.html

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