The least common multiple of a and b is 12, and the least common multiple of b and c is 15. What is the least possible value of the least common multiple of a and c?<spanclass=′latex−bold′>(A)</span>20<spanclass=′latex−bold′>(B)</span>30<spanclass=′latex−bold′>(C)</span>60<spanclass=′latex−bold′>(D)</span>120<spanclass=′latex−bold′>(E)</span>180 AMC 82016 AMC 8number theoryleast common multiple