Prime numbers are one thing. Combining them with counting principles is an entirely different thing together.
Let’s say I have a two-digit number. All you know is that both digits are prime numbers. So you could have 53, 37, etc. But how do you actually count up all these numbers without having to list them all out (as many, I know, are tempted to do).
The way we do so is by counting the total number of possibilities for each case. In other words, how many different prime numbers can go in the tens place and how many can go in the units?
Well, the single-digit prime numbers are 2, 3, 5, and 7. You can’t include 11, 13, and so on, because these numbers themselves are two-digit numbers, and in both cases, both of those digits are not prime (‘1’, though many mistakenly assume it is, is not a prime).
Since there are four possibilities for the tens and four for the units, we get a total of sixteen different two-digit numbers.
Okay, now let’s up the stakes a little with this week’s challenge problem!
If you have any questions/comments/queries/conundrums/dilemmas, feel free to leave them for me in the comment box below! 🙂
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About Chris Lele
Chris Lele is the GRE and SAT Curriculum Manager (and vocabulary wizard) at Magoosh Online Test Prep. In his time at Magoosh, he has inspired countless students across the globe, turning what is otherwise a daunting experience into an opportunity for learning, growth, and fun. Some of his students have even gone on to get near perfect scores. Chris is also very popular on the internet. His GRE channel on YouTube has over 10 million views. You can read Chris's awesome blog posts on the Magoosh GRE blog and High School blog! You can follow him on Twitter and Facebook!
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