How much 90 percent vinegar do you have to add to a gallon of 5 percent vinegar to get 20 percent vinegar

I've been trying to solve this problem forever and I just can't do it. I emailed my instructor and she sent me the equation

0.05+0.9x/1+x=0.20

I thought from there I would be able to solve it, but I'm confused about the rules of eliminating things and what I should do first. Am I allowed to subtract the 0.05 from the 0.20 first? Or is do I need to multiply by the reciprocal? I'm really confused, please help. And I would prefer if I wasn't given the answer, I want to understand the concept so I can do it for myself.