Hamburger sells for 3 pounds for 6 dollars. if samantha buys 10 pounds of hamburger how much will she pay

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