The price of petrol rises from Rs. 80 to Rs. 100. What is the percentage increase in price?

Step-by-step percentage evaluation:
1. Find the absolute increase in price:

$$\text{Increase}=100-80=\text{Rs. }20$$

2. Calculate the percentage increase based on the original price (Rs. 80):

$$\text{Percentage Increase}=\frac{\text{Increase}}{\text{Original Price}}\times 100\%$$

$$\text{Percentage Increase}=\frac{20}{80}\times 100\%=\frac{1}{4}\times 100\%=25\%$$