a. As described in the problem you will be paying in the bill $0.15 per minute which means you have a linear relationship where both the total cost of the bill and the minutes will grow at the same rate (the price of the minute times the minutes) like this
[tex]C(t)=0.15t[/tex]
where C(t) is the total cost of the cell phone bill and t will be the time in minutes
b. The domain of the function will be the values that we can enter to the function. It is defined in the problem that this cost of the minutes its only up to 600 minutes so there is our limitation for the values that will enter the function. The domain will be between 0 and 600 both included because if he calls 0 minutes the bill will be 0 and if he calls 600 he would pay 0.15 for this last minute as well.
[tex][0,600][/tex]