Feature

Leave the calculator alone to deduce the missing digit

The key to a speedy solution is to realise that all factorials greater than 5! must be divisible by 9 (because 3 and 6 are factors of 6! and 3x6=18, which is a multiple of 9).

Now, it is well known that every number divisible by 9 has its digit-sum also divisible by 9. So we simply need to add up the eighteen digits that have been given, to arrive at 52. Then, the unidentified, leftmost digit must have been 2, to make a grand total - divisible by 9 - of 54.

Hence the full integer is 2,432,902,008,176,640,000

Try more puzzlers >>

Go to the homepage >>


Email Alerts

Register now to receive ComputerWeekly.com IT-related news, guides and more, delivered to your inbox.
By submitting you agree to receive email from TechTarget and its partners. If you reside outside of the United States, you consent to having your personal data transferred to and processed in the United States. Privacy

This was first published in May 2009

 

COMMENTS powered by Disqus  //  Commenting policy