Unknowns and balances
Strategy card
An equation is a balance: both sides stay equal.
- Step 1: Find the unknown (the box or letter).
- Step 2: Undo the operations in reverse order.
- Step 3: Do the same thing to both sides.
- Step 4: Check by putting your answer back in.
Tip: Substitute your answer back in. It's the quickest check.
Worked example
What number goes in the box?
□ − 9 = 3 × 10
- A: 42
- B: 21
- C: 45
- D: 39 (the answer)
- E: 46
Why: 3 × 10 = 30. The box minus 9 is 30, so the box is 30 + 9 = 39.
Your turn
Your turn 1 of 3
What number goes in the box?
□ − 7 = 2 × 10
Your turn 2 of 3
What number goes in the box?
5 × □ + 8 = 38
Your turn 3 of 3
Use the rule. What is the answer?
a ◆ b means 5 × a + b
6 ◆ 3 = ?
Keep practicing
Grown-ups: Ask: "What would you do to undo that?" Parent guide
Next lesson: Arguments and evidence