Average

The average mass of some boxes was 45 kg. When 4 more boxes each of mass 46.5 kg was added, the average mass of all the boxes became 45.5 kg. How many boxes were there in the end?

The average mass of some mangoes was 672g. After 2 mangoes each of mass 700g were eaten, the average mass of the remaining mangoes became 658g. How many mangoes were there at first?

Casey spent an average of $2.80 on some packets of chips. After he bought 2 more packets that cost $4.80 each, the average cost became $3.20. How many packets of chips did Casey buy altogether?

The average math quiz score of a group of students was 90 marks. After 4 students who scored an average of 86 marks left the group, the average score of the remaining students became 94 marks. How many students were there in the group at first?