15.(a)The monthly cost of local calls on a mobile phone is $8 plus 9 cents per call. Write an expression for the total cost, in dollars, of r local calls in x months. 15.(b)The cost of hiring a bicycle is made up of two parts. There is a fixed charge of $10 and a charge of $5 for each day that the bike is hired. Write an expression for the total cost, in dollars, of hiring a bike for n days.​