If x is the number of berries you can buy with 1 dollar,
1 pound of blueberries = $4.00
x pound of berries = $1.00
Lets set up a proportion- because these are directly proportional- when there is more pounds of berries, it will cost more.
Cross multiply
4x=1
Divide both sides by 4 to isolate x
x=1/4 or $0.25