For example: 51 is 17% of what number.
17% times number = 51 or 17% * number = 51 (* = times).
Convert 17% to a fraction, 17/100. Now we have 17/100 * number = 51.
 
Divide both sides by 17/100 to cancel out the 17/100 on the left. You should get number = 51*100/17.
 
When you divide by a fraction you flip it over and multiply by it instead. So right hand side gets multiplied by 100/17.
 
The number = 51*100/17 = (51/17)*100 = 3*100 (since 17 goes into 51 3 times and multiplication and division are on the same level so you can do them in any order)
 
The answer is 300.