Mona is 20% younger than Neetu. By how much percent is Neetu older than Mona ?

Lets take Neetu's age to be 100 years.

Mona is 20% younger than Neetu.

∴ Mona's age = 100 - 20% of 100

= $100 - \dfrac{20}{100} \times 100$

= $100 - 20$

= $80$

Mona's age is 80 years and Neetu's age is 100 years. Then we can say that Neetu's age is 20 years more than Mona's age.

Percentage by which Neetu is older than Mona = $\dfrac{\text{Neetu's age - Mona's age}}{\text{Mona's age}} \times 100$%

= $\dfrac{(100 - 80)}{80} \times 100$%

= $\dfrac{20}{80} \times 100$%

= $\dfrac{1}{4} \times 100$%

= $\dfrac{100}{4}$%

= 25%

Hence, Neetu is older than Mona by 25%.