Find absurdity in seemingly correct proof that $4=5$

Question

I saw a proof for $4=5$, which is quite simple.

It starts with $$20 - 20 = 25 - 25 \tag1$$

$$\implies 4\cdot(5-5) = 5\cdot(5-5) \tag2$$

Cancelling out $(5-5)$ from both sides:

$$\implies 4 = 5 \tag3$$ Hence proved.

Which step is wrong? All steps seems legitimate.

Answer 1

In equation $(3)$, you divided both sides by $(5-5)$, which is equal to $0$, and clearly, you cannot divide by $0$. Hence, equation $(3)$ is illegal and the proof is false.