Tilly started writing down numbers as she counted them out, “1, 2, 3, 4 …”
She was interrupted when she was writing down the 33rd 3.

What number was she in the middle of writing?

Scroll down for the answer!


brain icon


Brainteaser answer

Tilly would have been writing 138.

By the time Tilly gets to 100, she will have written twenty 3s: ten of them in the units (3, 13, 23 …) and ten of them in the tens (30, 31, 32 …). It will take three more 3s (103, 113, 123) before she needs to write any 3s in the tens. That leaves ten 3s to write, which you can easily count out.

For more brainteasers and puzzles for kids, subscribe to Double Helix magazine!

Subscribe now! button

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

By submitting this form, you give CSIRO permission to publish your comments on our websites. Please make sure the comments are your own. For more information please see our terms and conditions.

Why choose the Double Helix magazine for your students?

Perfect for ages 8 – 14

Developed by experienced editors

Engaging and motivating

*84% of readers are more interested in science

Engaging students voice