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! 🙂

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 8 million views.

You can read Chris's awesome blog posts on the Magoosh GRE blog and High School blog!

3 Responses to “SAT Video Friday – Prime Times”

1. Ibrahim says:

Hey 🙂

Great video! 🙂 I am practicing the questions on Magoosh as well as on Khan Academy. I would like to know which level of Math we need to know for common SAT test? SAT math level 1,level 2 or level 3? 🙂

Also are there any tips which will help in pacing? I am lacking skills in time management on SAT practice tests. I have SAT on 6 December. 🙁

• Chris Lele says:

Ibrahim,

For SAT common test, you will only have to know the basics of geometry, algebra, and number properties. No trigonometry,etc. You don’t even have to know logarithms. The test is tough because the writers are very good at making things tricky :).

As for pacing, it is important to remember that the following sections are staggered in terms of difficulty. In other words, the higher the number the question, the hard it is:

1. Sentence Completions
2. All math (on the Grid-in, the difficulty “resets” to easy beginning with question #9)

3. All grammar, except for the paragraph improvement (it “resets” at question #12 in the long writing section).

Knowing this will allow you to skip some of the harder questions, since each question on the SAT is worth the same number of points. There is no need to fret over a tough question. Move on to easy/medium questions and come back to it if you have time.

In general, knowing when to “let go” of a question is also important. You’ll save yourself from burning precious time, and you’ll also allow yourself to see the question afresh, which often helps you get the answer more easily the second time around (assuming you have time to come back to the question).

Hope that helps, and good luck on the 6th! 🙂

• Ibrahim says:

Thank you so much Sir. I only have Magoosh account to prepare from. I don’t attend any classes or have any books for it. 😀 So I am a bit nervous. I gave a practice test for maths and got score of 540. I really wanted to cry that time 🙁 😛 . I could have solved most of them but the time was running and that made me really anxious.

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