Level 3
The average number of stickers that Adam and Bryan have is 30.
The average number of stickers that Bryan and Chris have is 50.
The average number of stickers that Chris and Adam have is 40.
Level 3
The average cost of some books is $75. When another 3 books are added, the total cost increases by $190. The average cost becomes $70. How many books are there in the end?