Price of petrol was Rs p per litre last month. Price of petrol now is Rs (p − 5) per litre.
The price of petrol per litre decreased this month by Rs 5 than its price last month.
Step 1: Last month, the price was (p) rupees per litre.
Step 2: Now, the price is (p - 5) rupees per litre.
Step 3 (Idea): Amount of decrease = old price − new price.
Step 4 (Write it): ( ext{decrease} = p - (,p - 5,))
Step 5 (Expand): (p - p + 5)
Step 6 (Simplify): (0 + 5 = 5)
Conclusion: The price has decreased by (oxed{5}) rupees per litre.