There are four types of number machines: $$A$$, $$B$$, $$C$$, $$D$$.  Device A: Add $$3$$ to the input number;  Device B: Divide the input number by $$3$$;  Device C: Subtract $$3$$ from the input;  Device D: Multiply the input number by $$3$$.  These devices can be connected. If the device $$A$$ is followed by the device $$B$$, it is written as $$A-B$$. For example, input $$6$$, $$6 + 3 = 9, 9 \div3 = 3 $$. Thus the output would be $$3$$. If you input a number in the device $$B$$-$$A$$-$$D$$-$$C$$, and get $$60$$, the input number was ． 
Overseas Competition->Knowledge Point->Calculation Modules->Operations with New Definition
$$B$$-$$A$$-$$D$$-$$C$$ has the rule: divide the input number by $$3$$; add $$3$$ to the input number;~multiply the input number by $$3$$; subtract $$3$$ from the input. So the inverse operation gives $$((60+3)\div3-3)\times3=54$$. 