Idea 18of 23Builds onWork backwards to the unknown
Undo a take-away
How do you solve an equation with a take-away in it?
Today you'll discover
- how a bar shows the whole and its parts
- when to add and when to subtract
- how to check it
Think firstSam gave away 12 and has 30 left
How many stickers did Sam start with?
Decide first, then press Next. Find the whole, then the parts.
A long bar marked x, the stickers Sam started with, over two pieces: 12 given away and 30 left. Then the equation x − 12 = 30, the working x = 30 + 12 = 42, and the bar filled with 42 and the check 42 − 12 = 30. The last step shows a bar of 45 marbles over a missing piece taken out and 20 left.
See the whole ideathe big idea and the key word, in one place
The big idea
A bar model shows the whole and its parts: the parts always add up to the whole.
Compare themx − 12 = 30: the whole is missing, so add: 42. 45 − x = 20: a part is missing, so subtract: 25.
Easy to get wrongA take-away in the story does not always mean subtract to solve it. Draw the bar first.
Key word
- bar model
- a drawing of a whole as a long bar, with its parts as pieces end to end
Ready to use it?
- Solve take-away equations with a bar model.