What's new

The difficult dial

By David, 4 September 2019 Brainteaser

While working on an old weaving machine, Aisha spied a dial that needed repairing.
The dial had numbers counting upwards, evenly spaced around the outside. Although she couldn’t see the whole dial, she could see that the number 6 was directly opposite 17.
Can you work out how many numbers there are on the dial?

Scroll down for the answer!

 

brain icon

 

Brainteaser answer

If the numbers are evenly spaced, there will be the same amount between 6 and 17, whichever way you go.
There are ten numbers between 6 and 17, so there must be ten between 17 and 6. Adding in both 17 and 6, there must be 22 numbers in total!

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

Subscribe now! button

3 comments

  1. 0 is a number. most dials start at 0. so there are 6 numbers below the 6, and 6 numbers above the 17.

    so the biggest number is 23.

    therefore there are 24 numbers

      Reply
    1. Hi Simon,
      Starting at zero is a good point! but I don’t think you’re right.

      You have to turn the knob 11 notches to get from 6 to 17 in ascending order. since the two numbers are directly opposite, you need to pass the same number of notches if you turn the opposite way too.
      starting at 6, we turn 11 notches and end on 17:
      5, 4, 3, 2, 1, 0, 21, 20, 19, 18, 17.

      So the largest number is 21, and there are 22 numbers on the knob!

        Reply
  2. sorry , you are right. i went the wrong direction!!

      Reply

Leave a Reply

By posting a comment you are agreeing to the Double Helix commenting guidelines.

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

Double Helix Magazine ad

Give the gift of science this holiday season with a discount subscription to Double Helix! Hurry as it’s a strictly limited offer! Subscribe online today.