The problem below (Moscow Puzzles #313) can be solved in more than 4 ways,
(each d1 by itself) - using different approaches.
Find the number t and the digit represented by k in:
List your ways of solving it.
Mod 10, the equation becomes 9*t^2 = 4
Then t^2 must equal 6 mod 10,
so t can only be 4 or 6.
Substitute both values in [3*(230+t)}^2 and evaluate.
This leads to the result that t can only be 4