Windows 10: I have 6GB of RAM installed but only 3.24GB is usable

I recently added another DDR2 RAM with 2GB alongside my other 2 2GB RAMs.However, Windows only uses 3.24GB of them.Specs:Computer: Dell Vostro 200 Slim towerOS: Windows 8.1 x64 The issue persisted with other versions of WindowsDDR2 RAM slots, 3 RAM sticks each one containing 2GB. 800 MHzCPU: Intel Core 2 Duo e6300 1.87 GHz

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Windows 10 shows Installed RAM 24GB (16GB usable)

I've upgraded from Windows 7 professional to Windows 10 professional. I purchased the PC with Windows 7 Home Premium but later updated to Windows 7 Professional. When I was using Windows 7 Pro the system recognized and used the 24GB RAM I've installed. Since
upgrading to Windows 10 Pro it shows that only 16GB RAM is usuable of the 24GB installed. I've tried using the msconfig option and it shows maximum memory to be 24GB.

I need help.

 

i inserted new RAM [32GB] but rather 32GB shows usable 24GB is RAM mal

i inserted new RAM [32GB DDR2 used chips ECC] but rather 32GB shows usable 24GB is RAM malfunction????? on boot i get error slot 2...well?

 

I have 6gb of ram but only 4.96 is usable

I have 6gb of ram but only 4.96 is usable, i am using windows 10 64 bit , can this be fixed, and how??