Which of the following is the equation of a direct variation that has a constant of variation equal to - 1/2?
a. y = x - 1/2
b. 1/2- y = x
c. y = -2 x
d. y = -1/2 x

Two farmers pay a total rent of $140 for a pasture. the first farmer placed $4 oxen in the pasture, and the second farmer placed $30 sheep in the pasture. the farmers split the rent based on the proportion of the pasture eaten by their own animals that month. how much should the first farmer pay, if 1 ox eats as much as 10 sheep in a month?