It is perhaps simplest just to translate the given English sentence into a mathematical sentence.
... 20% ... ... of ... 57 ... is ... what number
... 20/100 ... × ... 57 .... = ... what number . . . . . . . . translation to math sentence
... 11.4 = what number . . . . . . . . carry out the multiplication
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If you're asked to write this as a proportion, you can write it like this:
You can solve this proportion by multiplying by 57 (the denominator under "what number"). Doing so gives you the same arithmetic sentence as shown above.