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?
2 × □ + 5 = 15
- A: 5 (the answer)
- B: 3
- C: 7
- D: 8
- E: 2
Why: Undo the steps: 15 − 5 = 10, then 10 ÷ 2 = 5.
Your turn
Your turn 1 of 3
Use the rule. What is the answer?
a ◆ b means 2 × a + b
5 ◆ 6 = ?
Your turn 2 of 3
Use the rule. What is the answer?
a ◆ b means 3 × a + b
5 ◆ 4 = ?
Your turn 3 of 3
what is 8x + 4?
If 4x + 4 = 16,
Keep practicing
Grown-ups: Ask: "What would you do to undo that?" Parent guide
Next lesson: Arguments and evidence