I use this proportion as a memory aid:

You put the numbers next/close to "%", "is", and/or "of" in this proportion and solve for the unknown.

Some examples:

1) What is 35% of 220?

Plug in 35 for "%", 220 for "of", and a variable for "is":

2) 42 is what percent of 840?

Plug in 42 for "is", 840 for "of", and a variable for "%":

3) 36 is 12% of what number?

Plug in 36 for "is", 12 for "%", and a variable for "of":

HTH.

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