Estimate the Number of Credit Card users in India.

This is a Fermi problem question, named after the physicist Enrico Fermi, designed to test an interviewee's problem-solving skills, particularly as they pertain to making good approximations with limited data. The fundamental concept behind this question is to take the broader problem and break it down into smaller, more manageable pieces.

Primarily, to answer this question effectively, you should:

1. Identify the total population of India.
2. Assess the percentage of India's population that is of an age eligible to own a credit card.
3. Estimate the proportion of those eligible who actually own a credit card, taking into account factors such as economic disparities, literacy, awareness of financial products etc.
4. Make a rational multiplication to arrive at your final answer.

Remember, this question isn’t about the exact number, but your reasoning and estimating ability.

India's population is approximately around 1.3 billion. Let's assume that around 70% of that number is of eligible age to own a credit card, which would be about 910 million.

We have to account for the fact that India has a vast economic divide and not all citizens have access to a credit card. If we consider that about 20% of the eligible population might own a credit card, this would bring us to an estimate of about 182 million credit card users.

The total population of India is about 1.3 billion. Let's assume, conservatively, that about 65% of the population is old enough to own a credit card - that would give us a pool of about 845 million people.

Now, we need to account for factors such as income disparity, rural versus urban spread and financial literacy. Assuming only about 25% of this eligible population actually holds a credit card, we can estimate there are approximately 211 million credit card users in India.