Sunday Times Teaser 2584 – Truncation
by Andrew Skidmore
Published: 1 April 2012 (link)
Callum is learning about squares and primes. He told me that he had discovered a four-figure square that becomes a three-figure prime if you delete its last digit. I could not work out his square from this information and he informed me that, if I knew the sum of its digits, then I would be able to work it out. So I then asked whether the square had a repeated digit: his answer enabled me to work out his square.
What was it?