How to Multiply Two Numbers If You Only Know How to Multiply and Divide by Two
Multiply any two numbers knowing only how to double and halve, an ancient column trick that feels like a magic shortcut hiding in plain sight.
This is a method I read in a book some time ago, and beyond its limited practical use I cite it as a curiosity.
I'll explain with an example: Suppose I want to multiply the numbers 57 and 31.
Step 1) To begin, we make two columns that we'll call A and B. Column A will be headed by 57 and column B by 31.
Step 2) In column A we'll divide the previous number by two, while in column B we'll multiply the previous number by two until we fill the table. Here's how it should look.
Step 3) Next we identify the even numbers in column A and cross them out, along with their corresponding values in column B.
Step 3) Finally we add up all the values NOT crossed out in column B and obtain the final result of 1767.
Well, by now some will say "you also need to know how to add!"… and yes, you also need to know how to add 🙂
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