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It’s a pretty common question we GMAT teachers get: “Can we go over combinatorics?” To which my answer is usually a barely contained sigh.
Are you ready for a challenge? Try to solve the following question in under two minutes:
How many different positive divisors does the number 147,000 have?
If you feel like two minutes are not nearly enough to solve the problem, you’re not alone. Even the most seasoned GMAT veterans might find the problem challenging, as it requires a deep level of understanding of two mathematical concepts: Divisibility and Combinatorics (just a fancy word for ˜counting’).
If I replaced the number 147,000 with the number 24, many more people would be able to come up with an answer:
You could just pair up the divisors (factors) and count them. Start with the extremes (1×24) and work your way in:
A quick count will show the number 24 has exactly 8 different positive divisors.
The number 147,000 will have many more positive divisors “ too many to count This is a strong indication that we will need to use combinatorics.
Divisibility: Any positive integer in the universe can be expressed as the product of prime numbers.