Reasoning · Grade 3-1 Three-Digit Calculation

Problem

Add and subtract by splitting numbers

Work each sum the way its worked example does. Move part of one number across, or round then correct, so one side lands on a multiple of 100. No column addition, and the total must not change. Find 496 + 145, 294 + 547 and 236 + 197.
Your answer
How to solve
Strategy Organize Information in More Ways — Each sum is re-organized (a number is split and part of it is shifted, or a number is rounded) into an easier related sum that has the same total but a round number in it, so I can add in my head. The total never changes because whatever I add to one addend I take away from the other.
1STEP 1

Make 496 into 500

Passing 4 from 145 makes 496 into 500: 500 + 141 = 641.

496 + 145 = 500 + 141 = 641
2STEP 2

Make 294 into 300

Passing 6 from 547 makes 294 into 300: 300 + 541 = 841.

294 + 547 = 300 + 541 = 841
3STEP 3

Round 197 up to 200, then correct

Rounding 197 to 200 then taking 3 back gives 436 − 3 = 433.

236 + 197 = 236 + 200 - 3 = 436 - 3 = 433
Answer
641, 841, 433
Each result is a three-digit number a bit larger than the bigger addend, which is what we expect when adding a few hundred. Quick column-addition checks confirm 641, 841, and 433, and all stay under 1000 as expected.
Takeaway

Make one number round first, and a 'hard' three-digit sum turns into one you can do in your head!

  • Make 496 into 500
  • Make 294 into 300
  • Round 197 up to 200, then correct