C(d) = (22-6*2.50) + 2.50*d = (22-15) + 2.50*d = 7 + 2.50 *d dollars. Here 22-6*50 = 22-15 = 7 dollars is the constant part of the cost. Sometime, they call it "the cost to turn on the counter". 2.50*d is the variable part of the cost. ANSWER. The cost function is C(d) = 7 + 2.50 *d dollars, where d is the travel distance.