Generaly, there are two IMAC 5k 27 2015 models, called "late 2015" and "mid 2015". On the following page you will find instructions on how to determine the exact model/serial number of the Mac used by the customer. If it is a late 2015 model, the memory HyperX Impact (1 x 8GB, DDR3-1866, SODIMM 204) should be compatible. If the model is "mid 2015", then Crucial CT8G3S160BM (1 x 8GB, DDR3-1600, SODIMM 204) should be compatible.