I have a nvidia geforce gt 820m and an i5 processor and an 8gb RAM fortnite is so playable and if so how should I adjust the graphics?
First on very low… Then you feel slow, until you have reached the "acceptable" limit of FPS. Of course, this limit is subjective.
By no means Fortnite, at least in HD or higher, with this outdated graphics card playable. You would have to play a lot less than HD to get at least a little FPS, which is not recommended. Your RAM is actually clear, if I knew what kind of i5 processor you have, I could also tell you if that's okay. However, your graphics card is as good as the worst of Nvidia that exists, making a fluid gameplay impossible.
Old and weak hardware.
Probably 720p on low playable. Think about 45fps.
The CPU is probably just a dual core.
