hardware - Can I add different size RAMs to iMac 5K? - Ask Different
KITONE 16GB KIT(2x8GB) Compatible for Apple DDR3 1333MHz PC3-10600 CL9 for Mac Book Pro (Early/Late 2011 13/15/17 inch), iMac(Mid 2010, Mid/Late 2011 21.5/27 inch), Mac Mini(Mid 2011) MAC RAM Upgrade - Newegg.com
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
iMac Intel 21.5" EMC 2428 CPU Fan Replacement - iFixit Repair Guide