you have one group of 3 lbs (six dollars)
you add another 3 lbs, total being 6 lbs (twelve dollars)
add another 3 lbs and you have 9 lbs (eighteen dollars)
now, do get that last pound cost you would divide six by three and find that one pound sells for two dollars.
add that 1 lb to your previous 9 lbs and you will get 10lbs costing $20
