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kittymaniac
18 is 45% of a number
18 is 45% of x
18 = 45% of x
18 = 0.45 * x

Now, to isolate x, you divide both sides of the equation by 0.45.

18 / 0.45 = 0.45*x /0.45
x = 40

So, 18 is 45% of 40. Hope this helps! :)
Part/Whole=Percent/100
Part=18
Percent=45
Whole= a variable that we are trying to figure out
so now its 18/x=45/100
Cross multiply and get 
45x=1800
Divide by 45 and get 40. 18 is 45%of 40