Kohl's Pennies, October 2011

This was an extra credit question given on a Nov. 2011 unit test. He was the only one to answer it correctly out of 100 students. Before his science teacher migrated to the 8th grade, the question was given each year until 2015.  Two off-the-charts girls finally got it in 2013, another one in 2014.

Question: Mr. Wright’s lab desk has a 2-liter beaker full of pennies.  Can you figure out how many pennies are in the beaker without counting them one-by-one?  Would you pay $20 for them?  Show your work. The following information should help:

1. The total mass of the beaker & pennies is 4.913 kg

2. Mass of the beaker is 556 grams.

3. All pennies are dated after 1982, so they are 97.5% zinc & 2.5% copper (a thin coating).  

4. The density of each penny is 7.143 g/cm3.

5. The volume of 100 pennies is 35.0 mL. 

Kohl showed his work and concluded that no, would not pay $20.  He had come up with 1,744 total pennies.  No deal.

I had written a vaguely similar problem on the board a week earlier. Kohl came in after school and worked on it for 30 minutes, covering an entire whiteboard with hieroglyphics and arrows to scattered numbers.  Eraser smudges were everywhere.  He wouldn’t allow any hints.  In fact, he refused them.  But just for a second he at least let his teacher frown at the spot where his math had meandered to the wrong path.  It was on the bottom-left side of his hieroglyphics.  

I kept telling him there was a bridge that would take him from one side to the other.  He'd move to the correct answer once it was found.  Kohl eventually found his bridge.  He was proud, yet humble.  Till this day, all these years later, I can still see Kohl's shy grin as he looked down at his sneakers.

After grading the tests, I counted those darn pennies one-by-one and came up with 1,742 pennies on my first try.  There were 1,745 on my second try.  There would not be a third count.  I reached for a beverage.  

Ever why some children are unforgettable?