windows 10 - Not all RAM available in OS?

I have 24GB of installed memory, 8GB 1600Mhz and 16GB 2133Mhz. However usable memory is at 6.96GB??

1. I have followed msconfig -> boot -> advanced options and have un-ticked "maximum memory" but i didn't help.


2. I have also tried leaving 16GB sticks but usable memory is still at 6.96GB.

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Further information about the system:
I am running AMD FX-8320 on "MSI 970 GAMING Socket AM3+ 7.1-Channel HD Audio ATX Motherboard".
RAM:

• Corsair Vengeance 16gb (2 X 8gb) Memory Kit Pc3-17066 2133mhz Ddr3 Dimm


• Samsung 2 x 4gb 1Rx8 PC3-12800U-11-13-A1 1600Mhz

BIOS does show a full amount of RAM.

I have installed new RAM today (16GB), hence the question.

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I am running windows 10 64-bit operating system, x64-based processor.

Answer

I have solved the problem by specifying amount of RAM in the "msconfig -> boot -> advanced options". After which i was able to achieve 15GB of usable RAM.

But, I was unable to use my 2x4GB sticks with 2x8GB sticks; for some odd reason GPU wouldn't start due to which i couldn't use computer.