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'Becky's Number Plumber' printed from

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This problem has been inspired by learners who were working on the Become Maths Detectives Project.

You may like to watch the video in the Become Maths Detective Project before having a go at this activity.

Becky created a number plumber which multiplies by $5$ and subtracts $4$.
Click on the picture to explore Becky's number plumber.

What do you notice about the units digit of the answer each time?
Can you explain why this happens?
Can you find any other numbers that work in a similar way?