solution

At point A, John would be willing to give up three gallons of gasoline for one bag of sugar and remain equally well off. Thus giving up two gallons of gasoline for one bag of sugar as in a move from point A to point B would make John better off (John would end up on a higher indifference curve, U2 J versus U1 J ). Maria would be willing to participate in the trade represented by the move from point A to point B since she is willing at point A to give up one gallon of gasoline in exchange for one bag of sugar. A move from point A to point B results in Maria getting two gallons of gasoline for one bag of sugar and makes Maria better off (she ends up on indifference curve U2 M versus U1 M at point A)