n = (10^10 x 9^9 x 8^8…3^3 + 2^2 + 1^1)^2

How many zeroes does n^2 contain?

First off, you need to recognize the pattern: every number has an exponent equal to itself, i.e. 10^10, 8^8 , etc. That means, that the (….) represents 7^7 x 6^6 x 5^5 x 4^4.

Now that we can survey the entire series (and I do recommend writing out the remaining numbers), notice is that 10^10 will yield 10 zeroes. The next step, though, is a little trickier. If you pair 5 and 2, you get 10. Therefore, each pair of 5 and 2 will yield a zero. There are five 5’s, yet only two 2’s. But notice how both 4^4 and 8^8 contain a bounty of 2’s (4^4 = 2^8 and 8^8 = 2^24), so we will have more than enough 2’s to pair with those five 5’s. So (5 x 2)^5 will yield a total of 5 more zeroes to ‘n’, giving us a total of 15.

Now to the very last part. The question is looking for n^2. You want to make sure not to square 15. See, when you have some number to an exponent and you are looking for the number of zeroes, you want to multiply by 2, since you are doubling the number of zeroes. Therefore, the answer is 30.

Now watch all of that explained in video form here:

If you have any questions or comments, please 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!

### 2 Responses to “SAT Video Friday – Zeroing in on the Answer”

1. Maliha Awan says:

Great explanation! Thanks so much for helping students. Could u plz recommend me the best book for sat preparation? With detailed explained answers n all kind of helpful material.. Plz mail me the name of that book. I’m really tensed. I can’t find a teacher for maths n I need to get self-prepared for the January test.. I’m preparing from the Barron’s SAT. Is that enough? Plz let me know if u have any good prep book in your mind. I’d b really grateful..

• Chris Lele says:

Hi Maliha,

A good combination of books is best! The Barron’s SAT is excellent for strategy and, to some extent, practice. But you will want to supplement that with the College Board’s Blue Book, which most consider the SAT prep bible. This book contains actual SAT tests. That way you can get the best sense of your score.

For a different take on SAT prep, you can also check out the Magoosh ebook, which is free:

http://magoosh.com/sat/2014/sat-ebook/

Hope that helps, and good luck 🙂

Magoosh blog comment policy: To create the best experience for our readers, we will approve and respond to comments that are relevant to the article, general enough to be helpful to other students, concise, and well-written! :) If your comment was not approved, it likely did not adhere to these guidelines. If you are a Premium Magoosh student and would like more personalized service, you can use the Help tab on the Magoosh dashboard. Thanks!