Okay, so for every 3 lbs of hamburger meat, Samantha has to cough up 6 dollars. But Samantha for some reason needs 10lbs.
First 3 lbs- 6$
Second 3 lbs- 6$
Third 3 lbs- 6$
That's a total of 18$ so far. But you still have one pound left. So you have to think, if 3 pounds is 6$, each pound must be worth 2$ each. So your last pound is worth 2$.
So she pays a total of 20$ for 10 lbs.
Hope that helps!