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1.
“The serial number of my new typewriter looks quite an ordinary 5-figure number to me, except its first and last digits are the same,” said Dad. “There’s more than that, Dad,” Mark told him. “It’s just three more than twice the square of its other three digits.” That did make it rather special, so what was the number?
Serial number 31253
Serial number 48424
Serial number 18961
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