Use ILP

The three children of a farm couple are sent to the market to sell 90 apples. Karen, the oldest, carries 50 apples; Bill, the middle one, carries 30; and John, the youngest carries only 10. The parents have stipulated 5 rules: (1) The selling price is either $1 for 3 apples or $2 for 1 apple. (2) Each child may exercise one or both options of the selling price. (3) Each of the three children must return with exactly the same amount of money. (4) Each childs income must be in whole dollars (no cents allowed). (5) The amount received by each child must be the largest possible under the stipulated conditions. Assume that the kids are able to sell all they have. Please Use ILP to show how the children can satisfy the parents condition.