Lesson 2 of 16
The order changes the answer
Show on a big screenA chit — a small bill — says 40 + 6 × 5. One reader gets 230. Another gets 70.
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Step 1 of 6
- The line: 40 + 6 × 5.
- Left to right: 40 + 6 = 46.
- Then 46 × 5 = 230.
- × first instead: 6 × 5 = 30.
- Then 40 + 30 = 70.
- Two answers: 230 and 70.
The rule
Both readers did real steps. Nobody made a mistake. They used different orders. An order is a plan for which step to do first.
A chit or a bill must mean one thing. So everyone needs one agreed order.
Try 20 − 6 ÷ 2. From the left: 20 − 6 = 14, then 14 ÷ 2 = 7. Dividing first: 6 ÷ 2 = 3, then 20 − 3 = 17.
In 6 × 2 + 4, both orders start with 6 × 2. So both give 16.
| The line | Left to right | × or ÷ first |
|---|---|---|
| 40 + 6 × 5 | 230 | 70 |
| 20 − 6 ÷ 2 | 7 | 17 |
| 6 × 2 + 4 | 16 | 16 |
Try the lab to finish this lesson.