The first one is just completely out, the store sold 25 stereos for 7 days is just too little per day. If the store sold 126 stereos for 7 days, meaning PER day, they would sell 18.
126/7= 18
they sell 18 per day. This is your answer. Since all the 7 numbers are 18, 18 is going to be your mean.
For the last two, if they sold 18 stereos on at least half of 7 days, that mean would be in between 18 and another number. And, if they sold 18 stereos on at least two of the days, I doubt the mean would be 18. Since the ratio is 2:7
