L5 · Addition and subtraction problem-solving clinic

Learning goal

“I can solve two-step problems and find unknown numbers. I plan first, estimate, and check my answer by undoing.”

Two-step problem — a problem that needs two operations in a row. The answer to step 1 feeds step 2.

Unknown value — a missing number in an equation, written as a blank box. You find it by undoing the operation.

Plan — saying what happens first and what happens second, before touching numbers.

Inverse check — proving an answer with the opposite operation: a sum is checked by subtracting; a difference is checked by adding.

Worked example

A school fair had 850 tickets. It sold 376 tickets on Saturday and 289 tickets on Sunday. How many tickets are left? Step 1 (plan): left = 850 - (sold in all). Road 1: add the sales, then subtract once. Step 2 (estimate): sales is about 400 + 300 = 700; left is about 850 - 700 = 150. Step 3 (compute): sales: 376 + 289 = 665. Left: 850 - 665 = 185. Step 4 (check): estimate said about 150-200. Inverse check: 185 + 665 = 850.

Check yourself — explain it

Adding the two sales first works because...

Paired problem — example still visible

A store had 925 pens. It sold 468 pens in the morning and 157 pens in the afternoon. How many pens are left? -> sold: 468 + 157 = 625; left: 925 - 625 = 300

Faded completion — last step is yours

"780 tickets, 245 sold Friday and 198 sold Saturday: sold in all 245 + 198 = 443; left: 780 - 443 = ___" -> 337

Ready to practise?

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

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