16. The cost (c) of a plane trip varies directly as the distance (d ) covered 17. The circumference (C) of a circle varies directly as the diameter (d). 18. The rate (R) of an automobile covering a distance (d ) varies inversely as the time (t). 19. In a triangle of constant area, the base (b ) varies inversely as the altitude (h). 20. The temperature (T) at which water boils varies inversely as the number of feet (h) above sea level.
