Friday Funny: Solution to Clocks, Calculators and Lightbulbs algebra maths puzzle

David Meego - Click for blog homepageA month ago I posted another picture algebra maths puzzles. This time it was the Clocks, Calculators and Lightbulbs algebra maths puzzle which looked simple but had more to it than you would initially think.

If you have not had a go at solving this puzzle and registering your answer on the poll; Stop reading the solution, click on the link above and go do it now. 🙂

The puzzle image is shown below:

And here is the “initial” solution:

  1. There are three clocks which total 21, so each clock is 7.
  2. There are three calculators which total 30, so each calculator is 10.
  3. There are two lightbulbs added and one subtracted, so each lightbulbs is 15.
  4. The final sum is 7 + 10 x 3(15) which equals 7 + 10 x 45.
  5. Doing Multiplication first gives 7 + 450.
  6. So the final answer is 457 …… WRONG.

Look closer at the image and you will notice the following:

  • The clocks hour hour is pointing to different hours.
  • The calculator digits are not always the same.
  • The lightbulbs have different number of “glow” lines.

So let’s start again, with the correct solution:

  1. The clock hour hands point to 9 + 9 + 3 = 21, so we must use the hour hands.
  2. The digits on the three calculators (1234) add up to 30, so we must use the sum of digits.
  3. The three bulbs have 5 “glow” lines each, so we must use these lines with value 3.
  4. In the final sum, the clock is 9, the calculator adds up to 9 and the bulbs have 4 lines.
  5. The final sum is 9 + 9 x 3(4×3) which equals 9 + 9 x 3(12) = 9 + 9 x 36.
  6. Doing the Multiplication first gives 9 + 324.
  7. So the final CORRECT answer is 333.

Where you really observant? Did you get it right? At the time of posting the solution, 48% of respondents on the poll were correct, which is pretty good.

Keep safe, well and sane.


This article was originally posted on

4 thoughts on “Friday Funny: Solution to Clocks, Calculators and Lightbulbs algebra maths puzzle

    • I still don’t ge the bulb part… Though i understand the lines but why are we using the value 3?? From where does 3 come??


      • Hi HY
        The sum that defines the lightbulb glow lines has a minus sign which cancel out two of the bulbs. So each glow line is 3. If it had been a plus sign then each glow line would be one. Is that what you missed?


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