Answer:
The 30% of a number can be calculated multiplying that number by 0.3, so, if we call the number we don't know x, we can write this equation:
0.3x = 12
And then solve it for x::
x = 12/0.3 = 40
So now we know that the number is 40.
We can replace the 40 in the equation to get if our answer is correct or not..
30% of 40: 0.3 * 40 = 12
And then, we see it's correct
