A number is decreased by 20%. If the resulting number is 800, the original number is :

1. 640

2. 960

3. 1000

4. 600

Let the number be $x$.

After a decrease of 20%, it becomes = 800

⇒ $x - \dfrac{20}{100} \times x = 800$

⇒ $x - \dfrac{20x}{100} = 800$

⇒ $x - 0.2x = 800$

⇒ $0.8x = 800$

⇒ $x = \dfrac{800}{0.8}$

⇒ $x = \dfrac{8,000}{8}$

⇒ $x = 1,000$

Hence, option 3 is the correct option.