You should be able to add a 16 GB stick for 24 GB total. But, sometimes mismatched RAM brands/models may not play well together. If they do work, 16 GB would run in dual-channel mode, while 8 GB would run in single-channel mode. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.
I have recently bought a g15 5525 and i have another ram stick that supports this device. the ram is ddr5. I was wondering if it was ok to combine the 8gb stick it came with originally with a 16gb ram. i have an extra slot for ram so i was wondering if it is possible to combine the two different sizes 
can you combine two different ram sizes in a g15 5525