Ram not used fully

Aug 23, 2018
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I have a real problem pls help.So i got a used PC,here are the specs:i7 4770,asus z87 deluxe quad,1x8gb ddr3,gtx 1050ti expedition.I think it is a good setup but games like fortnite show only 30FPS.I figured out it is the ram,because it show in windows,bios,and test programs 8GB but when i go in game to task manager it show that 50% of ram is used(2GB,so it means that it only registers 4GB of ram).I tried almost everything:I went to windows and turned off maximum memory,updated bios,inspected the ram,changed the ram slots but nothing worked.I suspect that it is the stick but I am not sure.If you have any advice for me please just say I can use all the help I get.