So for 3 of the days, there are 1 + 1 = 2 people eating. So for those days they use 3 * 2 = 6 plates. On 7 - 3 = 4 days the parents join them. On those days there are 2 + 2 = 4 people eating. They use 4 * 2 = 8 plates. So they use 8 * 4 = 32 plates. That means they use a total of 32 + 6 = 38 plates.
This justifies what answer for what question? Q & A:
Matt needs to buy new plates for his home. He only wants to do dishes once a week. Three days a week it is only him and his son eating and they use 1 plate each. On the remainder of the days, his parents join them and everyone uses 2 plates that day. How many plates does he need to buy?
38