Please help!! I can't figure this out

Bring the fraction a/a-4 to a denominator of 16-a^2

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?

If p(a) = 0.70 and p(b) = 0.20, then a and b are independent events if