So firstly, we want to know from which number can we take 30% and get 6. Well how about we turn that into an equation. So how about let's say that x is the number we are supposed to be looking for. 30% of x will be equal and then if you multiply 30% x 2 it will be 6. So we multiply each sides by 100 and then we divide 30 on each sides and we get: x=(6*100)/30 and that is equal to: x=20. So 30% out of 20 is 6.
(I will be honest about this. I kind of used someone's answer and changed it. But I hope it helps! :D)