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Debt Recovery

Stage: 3 Short Challenge Level: Challenge Level:1

Tina gives Tony 70p.

Let $x$ be the amount Tony owes Tina (which will be negative if Tina owes Tony).
Tony owes Tina 40p: $x=40$.
Tina borrows 50p from Tony: $x=-10$
Tony gives Tina 60p: $x=-70$.
So Tina owes Tony 70p.

This problem is taken from the UKMT Mathematical Challenges.