Level 3
The average cost of some books is $10.
After 2 new books are added,
it adds another $60 to the total cost.
As a result, the average cost of all the books increases to $20.
How many books are there at first?
Level 3
The average cost of some books is $30.
After 2 new books are added,
it adds another $10 to the total cost.
As a result, the average cost of all the books decreases to $20.
How many books are there at first?
Level 3
The average age of a group of teachers and lawyers is 40. If the average age of the teachers is 35 years and that of the lawyers is 50 years, what is the ratio of the number of teachers to lawyers?
Level 2
From January to August last year, Sherrie read an average number of 7.5 story books per month. She did not read any story books in the next 4 months. On the average, how many story books did she read per month last year?
