Cool trick! I see how it works by looking at [(a*10) + 5]^2 and expanding. That gives (a*10)^2 + 2*a*10*5 + 5^2 which is 100*a^2 + 100*a + 25 Or 100*(a^2 + a) + 25. Since a^2 + a is the same as a*(a+1), and the factor of 100 just moves that to the left of the 25, there’s hour trick. As advertised, it should always work!
