It is all a big math problem. Write down all the Number that match 54, then go from there.
99
9+9 = 18
99 - 18 = 81
98
9+8=17
98 - 17 = 81
96 = 81
95 = 81
There are only a few numbers that with the math given that any of these numbers will equal.
Also, notice that the numbers that match start with 9 go up by 9's 9, 18, 27, 36 etc.
They do throw the extra matches to keep you guessing though :-D
;-) get it????