If 1/x+ 1/y= 1/4 and 1/x-1/y=3/4, then x=
a) 1/4
b) 1/2
c) 2
d) 4
someone

Aparticular telephone number is used to receive both voice calls and fax messages. suppose that 25% of the incoming calls involve fax messages, and consider a sample of 25 incoming calls. (a) what is the expected number of calls among the 25 that involve a fax message?