Potatoes cost Janice $1 per pound, and she has $5

Lets assume, that Janice has $5 to spend on Potatoes or other items and cost of a pound of potatoes is $1.

Now assume, first pound of potatoes is worth $1.50 to Janice and she will purchase the pound of potatoes till the value of pound potatoes $1.50 or greater that the cost of potatoes that is $1.

Also when the second pound is worth $1.14 and the third pound is worth $1.05 then Janice will purchase these same as the value exceeded the cost of $1. If all remaining pounds are worth $0.30 then Janice will not purchase these because of the value is less than the cost. So, Janice will purchase $3 pounds of potatoes at total cost of $3.

If Janice has only $1 to spend on potatoes then she will purchase the first pound because it's worth $1.50 and it has cost only $1. She can purchase the second pound because it's worth is $1.14. She has now spent her whole income on potatoes. She would like to purchase the third pound because it's worth $1.05 but has no income to purchase them.

So, She will be buying only 2 pound of potatoes with the budget of $2 because the marginal value of first two pounds are greater than then $1 price of potatoes.