Level 3
The average cost of some books is $30.
When a book that costs $90 is added,
the average cost of the books becomes $40.
How many books are there in the end?
Level 3
The average cost of some books is $30.
When a book that costs $90 is removed,
the average cost of the books becomes $10.
How many books are there in the end?
Level 1
There are 3 boxes. The mass of Box A is 3.4 kg, the mass of Box B is 5.8 kg and the mass of Box C is 4.2 kg. Which box has its mass nearest to the average mass of the 3 boxes? Give the letter only for the answer.