Student's Written Calculations

I read an Analog story in the 1970's about a student who used a pencil and paper to figure out the math test question of the day and he finished the problem before anyone else in the class who were using their calculators to answer the problem.

The student learned this AMAZING TRICK from his grandfather.

Does anyone recall what this story is titled and written by which author?

The Cartoon "LUANN" has a similar theme in today's Sunday Paper - 1/25/15.