RAM upgrade considerations for iMac systems - CNET
Install memory in an iMac - Apple Support
Timetec 16GB KIT(2x8GB) Compatible for Apple DDR3 1067MHz / 1066MHz PC RAM for Mac Book (Mid 2010 13-inch), Mac Book Pro (Mid 2010 13-inch), iMac (Late 2009 27-inch), Mac Mini (Mid 2010)
Apple A1311 iMac 21" 2010 3.1GHz Core i3 12GB RAM 250GB HDD os10.13 Ra