We already know that 100 apples is the goal that Walter and his brother are trying to reach. The amount collected so far is 30, because Walter has a parcel of 30 apples. However, Dwight does not have any. We can then find how many are left with the equation 100 - 30, because we are subtracting what we already have from our goal. The answer is 70 and Walter has 70 apples left to collect to reach his goal.