91av

Some check!

Joe has been teaching Penny all about “checksums”, which have been used for years to check the accuracy of calculations.

To calculate the checksum of a number you add up its digits. If that total is a single digit then that is the checksum; if not, the digits in the total are added to produce a new total and the process is repeated until a single-digit answer is produced. That single digit is the original number’s checksum.

What is the checksum of the number (2¹⁰⁰ + 3²⁰⁰)³⁰⁰?

WIN £15 will be awarded to the sender of the first correct answer opened on Wednesday 7 October. The Editor’s decision is final. Please send entries to Enigma 1561, 91av, Lacon House, 84 Theobald’s Road, London WC1X 8NS, or to enigma@newscientist.com (please include your postal address).

Answer to 1555 Not a square: The 2-digit number is 71

The winner Terry Lovekin of Scarborough, Ontario, Canada