Idea 14of 23Builds onGuess, check and revise
Work backwards to the unknown
How do you work backwards from the total to x?
Today you'll discover
- which step to undo first
- what undoes adding and multiplying
- how to check forwards
Think first5x + 6 = 41: times 5, then add 6
Before the + 6, what number had x × 5 made?
Decide first, then press Next. Undo the last step first.
Three boxes in a row: x, a question mark, and 41, joined by arrows marked times 5 and plus 6. A curved arrow back marked minus 6 fills the middle box with 35; another marked divide by 5 fills the first box with 7. The bottom line checks: 7 × 5 = 35, then 35 + 6 = 41. The last step shows 6x − 4 = 50.
See the whole ideathe big idea and the key word, in one place
The big idea
Work backwards: undo each step with its opposite, last step first.
Undo, in reverse order
From 41 back to x.
- total41
- − 635
- ÷ 5x = 7
Where it goes nextPatterns & Rules did this to find which figure had 41 tiles: undo the rule, last step first.
Key word
- inverse operation
- the opposite operation, which undoes another: − undoes +, and ÷ undoes ×
Ready to use it?
- Solve an equation by working backwards.