Alright, this is simple. You know what the new number is, 54. You know the original number was decreased by 40%. Your looking for the original number, X. So set up an equation.
x-.4x=54
When X stands by itself, it's perceived to value 1.00
1.00-.4x=54
(1.00-.4 [40%]=.6)
.6x=54
Divide both sides by .6 to eliminate everything else.
.6x/.6=54/.6
You get 90. Your original number was indeed 90.
To check it, we can do 90/40%, or 90/.4=54
~Hope this helps!