2006 AMC 12A Problem 14

Two farmers agree that pigs are worth 300 dollars and that goats are worth 210 dollars. When one farmer owes the other money, he pays the debt in pigs or goats, with "change" received in the form of goats or pigs as necessary. (For example, a 390 dollar debt could be paid with two pigs, with one goat received in change.) What is the amount of the smallest positive debt that can be resolved in this way?

\mathrm{(A) \ } \$5 \qquad \mathrm{(B) \ } \$10 \qquad \mathrm{(C) \ } \$30 \qquad \mathrm{(D) \ } \$90 \qquad \mathrm{(E) \ } \$210

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.