The car’s speed first decreased by 20% and then increased by 20%. How much has the original vehicle speed changed?

In the task, it is required to determine the percentage of change in the initial speed of the car, when the speed first decreases by 20%, and then increases by the same percentage, that is, by 20%. Without loss of generality, we will take the initial speed equal to 1.
It would seem that first -20%, and then + 20%, and that’s it; we get the initial cost. However, the situation here is different. The fact is that when we find a reduced speed, we get 100% – 20% = 80% of the original cost, that is, we get a speed equal to 0.8.
By increasing this speed by 20%, we now get a speed equal to 120% of the speed 0.8. We have: 120% * 0.8 = 0.96.
Thus, the initial vehicle speed changed by 1 – 0.96 = 0.04. This means that it has decreased by 4%.
Answer: The original vehicle speed has decreased by 4%.