The Numberland News runs personal ads. 21 was looking for a new friend and put an ad in.
Two-digit, semi-prime, triangular, Fibonacci number seeks same. I’m a binary palindrome, what about you?
Will 21 find a friend?
A semi-prime is a number with exactly two prime factors, like 6. See this post for a definition of triangular number, this post for some hints on how to figure out a general formula for computing triangular numbers, and this one for the solution. Fibonacci numbers are discussed here. Finally, a palindrome is a number (or word, or phrase) which is the same forwards and backwards; a binary palindrome is a number which is a palindrome when expressed in base two.