Mental Column Addition

Mental Math · Axiom Academy

EXAMPLE Mental Column Addition Add a column of numbers by running a total down the list, then spot a pair that combines to a friendly number. Add this column of numbers mentally: 23 , 45 , 12 , and 38 . Ones digits of the highlighted pair: Nice work — you added a column of four numbers mentally two different ways and landed on the same total both times. Remember: Scan for a complement pair first: ones digits that sum to 10 (like 2 and 8 ) make a genuinely friendly, round subtotal. A running total always works: add one number at a time, left to right, keeping a mental sum as you go. Compensation still helps: rounding a number up (treating 38 as 40-2 ) and adjusting at the end is another valid path. Regrouping never changes the sum: pair the column's numbers however you like — you'll always land on the same total. You'll get to chain longer columns together in the activities and drills ahead.

This is the written version of the interactive lesson above. See the full Mental Math course.