Plz give a step by step two column proof. Thank you in advance. The first step should be the given. After that there should be 3 steps.

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?