Easy method is that /27 means 27 bits out of 32 are used as network addresses, and the remaining are used for hosts...
11111111 11111111 11111111 11100000 is a /27,and if you know binary, then you would know that 11111111=255 (1+2+4+8+16+32+64+128), and 11100000=32+64+128, which=224.
Burt