Quick Calculation: Make-a-Ten & Splitting

Split a near-hundred number into a round hundred minus the gap, then take the extra back: 98 + 45 = 100 + 45 − 2

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About This Practice

How do you work out 98 + 45 quickly and correctly? Round it up: 98 is only 2 away from 100, so split it as 100 − 2, work out 100 + 45 = 145 first, then take the extra 2 back to land on 143. Step through the buttons and watch the big hop (+100) and the small step back (−2) on the number line.

Step through 98 + 45: notice 98 is 2 away from 100, split it as 100 − 2, add the round number first (100 + 45 = 145), then subtract the extra 2 to get 143. A number line shows every hop and step of the rounding trick.

MediumEstimated Time: 12minmental mathmake a tenroundingsplitting numbersquick calculation

How to Play

  1. Look: which addend is close to a round hundred? By how much?
  2. Split: rewrite 98 as 100 − 2 and see the big hop on the line.
  3. Round: add the round hundred first — 100 + 45 = 145.
  4. Adjust: subtract the extra 2 — 145 − 2 = 143, one small step back on the line.
  5. Tap New Problem and practice until the trick becomes a habit.

Learning Objectives

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