Two fathers and two sons walk into an ice cream parlour. Each buys something for Rs 50, yet the total bill is only Rs 150. How is this possible?

Introduction / Context:
This puzzle looks like a contradiction in simple arithmetic. If two fathers and two sons each spend Rs 50, we might expect the total to be 4 × 50 = Rs 200. However, the problem states that they pay only Rs 150. Rather than being a question about discounts or mistakes, it is actually a family relationship puzzle that plays with how we count fathers and sons in a small three generation group.

Given Data / Assumptions:
• The phrase two fathers and two sons describes the group walking into the shop.
• Each of them buys something for Rs 50, so each person pays the same amount.
• The total bill mentioned is only Rs 150, not Rs 200.
• We assume the puzzle is correct and there is no hidden discount or error in calculation.

Concept / Approach:
The key idea is that the phrase two fathers and two sons does not necessarily mean four different people. It can describe three people in a family: one grandfather, his son and his grandson. The grandfather and the father are two fathers, and the father and the grandson are two sons. This arrangement produces exactly two fathers and two sons, but there are only three actual people. So the total cost is 3 × 50 = Rs 150, which matches the puzzle statement.

Step-by-Step Solution:
Step 1: Let us assume there are three generations in the family: a grandfather, his son and his grandson. Step 2: The grandfather is a father to the middle generation man. Step 3: The middle generation man is both a son (to the grandfather) and a father (to the grandson). Step 4: The grandson is a son to the middle generation man. Step 5: Counting roles, we have two fathers (grandfather and father) and two sons (father and grandson) but only three people. Each spends Rs 50, so 3 × 50 = Rs 150, which matches the total in the puzzle.

Verification / Alternative check:
To verify, we can list each person along with his roles: Person 1 (grandfather) is father but not son; Person 2 (father) is both father and son; Person 3 (grandson) is son but not father. That gives exactly two individuals who are fathers and two who are sons, with an overlap at Person 2. Since the total number of distinct people is three, and each spends Rs 50, the reported total Rs 150 is perfectly correct. No discount or error is needed to explain the amount.

Why Other Options Are Wrong:
• A special discount would change the effective price per person, but the puzzle says each buys something for Rs 50, indicating no price reduction.
• If one son forgot to pay, the story would be incomplete and misleading, and the description each buys something for Rs 50 would be false.
• A shopkeeper miscalculation turns the situation into a simple error rather than a logic puzzle, and the puzzle would not be interesting or unique.

Common Pitfalls:
Many learners immediately multiply 2 fathers and 2 sons as four separate people and focus only on the money, missing the family structure trick. Others may think there must be a discount without noticing the phrase each buys something for Rs 50. The lesson is to carefully question whether phrases like two fathers describe distinct individuals or overlapping roles in relationships. This is a common pattern in logical puzzles involving family terms.

Final Answer:
There are only three people in the group, a grandfather, his son and his grandson, so 3 × Rs 50 gives a total of Rs 150 even though there are two fathers and two sons by relationship.