How to figure out problems like 54 is 75%

You have to learn ho to translate some common words into algebraic expressions and equations.

Here are a few:

"is" means "equals". 
When you see "is", write an 'equals sign.  (  =  ).

"of" means "times". 
When you see "of", write 'times' or whatever symbol you like to use for it.

"percent" or the symbol " % " means "divided by 100".
When you see (a number) with 'percent' or ' % ' after it,
write (the number) 'divided by 100'.

A fraction-bar ( / ) and 'divided by' mean the same thing.
When you see either one, you can always write the other one if you want to.
Examples:
                  ' 1 divided by 2 ' means  1/2 .

                 47/54 means ' 47 divided by 54 '. 

Now we're ready to look at the example in your question:

                        54  is  75%  of  (answer) .

Translation:    54  =  75/100  times (answer)

Now you have an equation. Go to work !  Manipulate it so that
you have (answer) on one side of the equation, and everything else
on the other side.  You can do anything you want to one side of the
equation, but the rule is that whatever you do to one side, you must
immediately do exactly the same thing to the other side.
Here's how it works:

                       54  =  75/100  times (answer)

Multiply each side by 100 :    5400  =  75 times (answer)

Divide each side by 75 :        5400 / 75  =  answer

Do the division for the fraction, and you have    72 = answer

The answer might be a