Problem of the Week

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Pick a Prime

Posted: September 15, 2025

70 % of users got this correct

How many four-digit prime numbers exist of the form ab(ab+1)?
Please note that a is a digit (1-9), b is a digit (0-9), and ab + 1 is a two-digit number that may include zero. (a cannot equal zero to ensure a four-digit number). For example, if a = 1 and b = 6 then ab(ab + 1) = 1617.

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