For nikiyas plan, the value of the constant of the variation is

Based on the graph shown to the right, Nakia's  phone cost varies directly with the amount of time spent talking.

Mark’s cost does not vary directly with time spent talking because the cost of 0 hours is not 0 dollars.

For Nikiya’s plan, the value of the constant of variation is 15.

which represents Nikiya’s cost per hour.