GRE Question of the Day (January 17)

Last year, a coffee roasting company purchased new roasting machines to bring their total number of machines to 10. These 10 machines can roast 210 lbs of coffee beans in 3 hours. Before purchasing the new machines, the company could roast 84 lbs of beans in 2 hours. If all of the machines work at the same constant rate, how many machines did the company buy last year?