So in the above question we are asked that create a condition where grace, zane and rupert don’t get what they want. So we know zane wants either strawberry or lemon soda. But as rupert have already selected lemon we need that zane do not select strawberry. So it is necessary that grace select strawberry before zane. So 1st grace and get strawberry. Also Rupert wants grace go first if she let rupert go second, so if we want to prevent rupert going second we will let him go third.
In short, preclude or make all childrens unhappy by using grace in a good way.