16GB RAM to 4GB

According to the specs at the link below that laptop has 4 ram slots (2 user accessible and 2 not, I take that to mean you can get to the user accessible ones more easily !) and has 2 x 4GB ram sticks when new.
http://www.toshiba.co.il/en/discontinued-products/satellite-p875-31t/
Did you replace both of those with 8GB sticks ? Which ram slots is the present ram in and are they the same slots the original ram was in ?
Is your Windows 10 64 bit ?

 

How much ram is shown in the Bios ? If less than 16GB try setting the bios to default values and check ram amount again.

 

Go to control panel>system and security>system. Post a screenshot of what it says. Here is an example.

 

OK, if the bios shows only 4gig, that is all the operating system is ever going to see/use.
Some things to try;
1 Reseat all of the ram. Pull out each stick; blow the slot out with a can of air and reinstall
2 Have you loaded optimized defaults in the bios? If not, do so
3 Pull out all new ram and see if the bios shows 8gig [the original ram]

Do use caution when swapping/installing ram. Pull out the laptop battery and remove the A/C pw plug. Press the ON button 2~3 times; all residual pw is now dissipated. Now you can swap/reseat ram.