Let, I will buy x pounds Blueberries and y pounds Strawberries.
I need to collect at least 3 pounds of fruits.
It also given that I can not spend more than $21, that is .
If I will buy 4 pound Blueberries and 1 pound Strawberry, then x = 4 and y = 1.
Now, 4 + 1 = 5 > 3. First inequality is satisfied.
, Second inequality is also satisfied.