Did you know you can subtract left to right?

Investigate the methodology of the Japanese Soroban. It uses complements (1 is the complement of 9, 2 of 8, etc.) to do a subtraction problem like the ones you are demonstrating. I will do the first one: 8 - 5 = 3 (3 tens), 2 - 7 (Use complements - the complement of 7 is 3, add 3 to 2 to get 5 (the ones position) and then subtract 1 (1 ten) from 3. Answer: 25. Vedic arithmetic does the same thing. Check it out.

My son was subtracting this way in elementary school (1st/2nd grade). Hard for the teachers to allow "negative numbers" 😉

A second grader taught me this strategy he “invented” ;) @HDascheberg

@AnneHasse

@MrsStroh17

Awesome stuff!

Ah yes, good old James Tanton. I’ve spent a lot of time watching him on YouTube.

245-187 done also left-to-right but slightly differently: 245-100 = 145 (i.e., 2 hundreds minus 1 hundred, with remainder) 145-80 = (14-8) or 6 tens, keep the 5 remainder for 65 65-7 = 58 (by any chosen method)

Just thinking that requiring students to explain their methodology shall carry more credit than just the correct answer.

Neat! I did my own example. And I think this is often how I do it in my head, but didn’t realize how easy it would to put it in an organized fashion on paper.