L1 · From two-digit to three-digit strategies

Learning goal

“I can use my two-digit strategies - partial sums and counting on - to add and subtract three-digit numbers. The strategies do not change. Only the numbers grow.”

Strategy — a chosen way to solve a problem. You already own good ones from Grade 2.

Partial sums — adding place by place: hundreds with hundreds, tens with tens, ones with ones, then joining the parts.

Counting on — starting at the smaller number and jumping up to the bigger number to find the difference.

Sum — the result of an addition. (The result of a subtraction is the difference.)

Worked example

246 + 358. Step 1 (split by place): hundreds 200 + 300 = 500. Tens 40 + 50 = 90. Ones 6 + 8 = 14. Step 2 (join the parts): 500 + 90 = 590; 590 + 14 = 604. Say it plainly: this is the Grade 2 strategy with one extra part. The hundreds are just one more partial sum.

Check yourself — explain it

Partial sums give the right answer because...

Paired problem — example still visible

357 + 264: hundreds 300 + 200 = 500; tens 50 + 60 = 110; ones 7 + 4 = 11; join -> 621

Faded completion — last step is yours

"428 + 275: hundreds 400 + 200 = 600; tens 20 + 70 = 90; ones 8 + 5 = 13; join: 600 + 90 + 13 = ___" -> 703

Ready to practise?

10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.

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