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2.4 rounded is 2.

2.4 + 2.4 = 4.8, rounded is 5.

Therefore, 2 + 2 = 5.

I was taught this as a "mind teaser" in middle school once. It was supposed to be cute and fun, but just ended up confusing half the class.

Cute and fun, but also very real if you're a CPU: https://0.30000000000000004.com/

I wouldn't consider the brain teaser a result of FP error. But yes, something to consider anyway.

2 + 2 = 5, for sufficiently large values of 2 and sufficiently small values of 5.

Talking about undefined behaviour.