If the entire class lasts 7/8 of an hour, then it lasts 52.5 minutes as:
60/8 * 7 = 52.5mins or 52min 30secs
a) Then warm up and cool down lasts 3/10 of the class so this is:
52.5/10 * 3 = 15.75mins or 15min 45secs
b) Instruction is 1/5 of the class so 4/5 is not spent on instruction so:
52.5 / 5 * 4 = 42mins