Number S is obtained by squaring the sum of digits of a two digit number D. If difference between S and D is 27, then the two digit number D is:
There is following back-solving method to find an answer:
(b) satisfies the given condition, i.e. (5+4)^2–54=27
(the source: Winners’ Guide to GMAT Math – Part II)
But, can somebody advise me what is a straight-way-solution method, if any?