%24The%20cost%20per%20hour%20of%20a%20bike%20ride%20is%20given%20by%20the%20formula%3A%24%5C%20c%3Dx%5E2-15x+70%24%5C%5Cwhere%20%24x%24%20is%20the%20distance%20travelled%20in%20Km.%20Find%20the%20distance%20that%20gives%20the%20minimum%20cost.