Question

Which two-digit number is equal to the sum of its first digit plus the square of its second digit?

Answer 1

10x+y=x+y^2
=>9x=y^2-y
No. Is 78,since x&y belongs to {0,1,2,3,4,5,6,7,8,9}

Answer 2

Think of all the square numbers from 0 - 9 (as they are the only one digit number). 0, 1, 4, 9, 16, 25, 36, 49, 64, 81. Remove all one digit numbers and this leaves 16, 25, 36, 49, 64 and 81.

Now add the first digit to the number. This creates a new list with 17, 27, 39, 53, 70 and 89. And seeing as 89 is the only number to end with its original pre-square number, it is safe to say that it is the answer.

Hope it helps because I took a much longer path to figure this one out.

(P.S I know where you got this question)