Let y represent the original price of potatoes per pound
Since he purchased the potatoes at 40% off the original price, this can represented as y*(1-0.4)
Now the weight of potatoes he purchased is x pounds, the total cost of these potatoes can be represented as x*y*(1-0.4)
The total amount of money he had is $10
The amount of money left over can be represented by the expression =
10 - x*y*(1-0.4)
where "x" is the weight of potatoes in pounds and "y" is the original price per pound of potatoes