For each of the relationships described below, determine if the variables are directly proportional. Write a sentence or two for each to justify your answers.

a.) Total number of words a typist can type and the number of minutes spent typing if he or she types 72 words per minute.

b.) The temperature of an asphalt road at several times on a hot summer day and air temperature at those same times.

c.) Total number of miles you travel if you average 26 miles per gallon and the gallons of gas used.

d.) The total days of vacation earned after y years of work if you earn two vacation days for every three months you work.