From the information given,
Initial budget = $180
Amount per meal = $9
Cost of 20 meals per month = 9 x 20 = $180
This means that he has nothing left. If he would like to eat 28 meals out per month, the additional number of meals is
28 - 20 = 8
The cost of eating 8 meals is 8 x 9 = 72
Therefore, the additional amount that he needs to budget is $72
