Start by calling the unknown amount 'x'. We can then set up a relationship to represent the question:
[tex]213 = (30\%) \times x[/tex]
or
[tex]213=0.3x[/tex]
and then solve for x, by dividing both sides of our equation by 0.3
[tex]\frac{213}{0.3}=\frac{0.3x}{0.3}[/tex]
[tex]710=x[/tex]
213 is 30% of 710
To check: [tex]710 \times 0.3 = 213[/tex]
