Question

If 15% of x is 42, what is x?

Answer 1

"x" is 100%, and 42 is 15% of that, so, what's "x"?

$\bf \begin{array}{ccll} amount&\%\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ x&100\\ 42&15 \end{array}\implies \cfrac{x}{42}=\cfrac{100}{15}\implies x=\cfrac{42\cdot 100}{15}$

Answer 2

Write 15% as a fraction= 15/100= 3/20 times x=42 thats one equation. 
3/20x=42
x= 42* 20/3 (reciprocal)
x=14*20
x=406