Four pages on one strategy: change the numbers so one becomes a friendly hundred, without changing the total. Page 1 asks students to reason before adding. Page 2 shows both ways to do it, giving ones from one number to the other and adding too much then taking it back. Page 3 practises with help, checks true-or-false changes and asks when the strategy is worth using. Page 4 compares it with the column method and asks for a sum of the student's own.
Put 398 and 205 on two plates and move 2 cubes across: ask "Did the total change?" Then try 437 + 199: "What if I add 200 instead? How much too much?" Page 3's "when does it help" is a good class discussion: not every sum needs it.
Ask your child to make one number a hundred before adding, and to tell you what they did to the other number. Money works well: 99 cents is a dollar take away a penny.
This pack supports the following Common Core State Standards for Mathematics:
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Adding three-digit numbers in your head by changing them into friendly numbers that keep the total the same. Students move ones from one number to the other, add too much and take it back, check whether a change keeps the total and choose when it helps.
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