Circle Rotation Paradox

[P6 Maths and beyond: Circles]

The following question is a tricky question from 1982 that a lot of people got wrong.

It's deceptive (and tempting) to use the large circle B as the point of reference when answering this question. Sure, the circumference of Circle A makes three rounds around Circle B, but that is in relation to the circumference of Circle B as opposed to position of Circle A.

In Maths, we must be rigorous with the question and the answer. The question specifies "How many times will Circle A revolve in total", which means the point of reference we must look at is Circle A and how many times it completes a revolution. (in this case, the number of times a point of reference cycles 360 degrees around itself)

However many times it may revolve around Circle B does not matter. That simply shows the ratio of A's circumference to B's circumference. It does not solve the question of [# of Circle A revolutions] even though it's intuitive to think so.

Be wary of counter-intuitive questions like these. They are especially popular in both PSLE exams, and Express test papers.

Featured Posts
Posts Are Coming Soon
Stay tuned...
Recent Posts
Archive
Search By Tags
No tags yet.
Follow Us
  • Facebook Basic Square
  • Google+ Basic Square

© Mind Centre for Learning Pte Ltd

Mon - Fri 2pm - 9pm, Sat 9am - 7.30pm, Sun 12.30pm - 7.30pm | 9653 0629, 8715 1708, 6285 5891, 9838 8821 |

  • w-facebook