In a bookstore, the number of notebooks was ²₅ of the total number of notebooks and correction tapes together. Charlie sold the notebooks and correction tapes in sets of 1 notebook and 6 correction tapes. After selling all the correction tapes, he still had 21 notebooks left. How many correction tapes were in the bookstore at first?

	Notebooks	Correction Tapes
Before	2x2 = 4 u	3x2 = 6 u
Change	- 1x1 =  - 1 u	- 6x1 = - 6 u
After	21	0

Since all the correction tapes were sold, the number of correction tapes at first and the number of correction tapes sold were the same.  Make the number of correction tapes at first and sold the same.  LCM of 3 and 6 is 6.

Number of notebooks sold
= 4 u - 1 u
= 3 u

3 u = 21

1 u = 21 ÷ 3 = 7

Number of correction tapes at first
= 6 u
= 6 x 7
= 42

Answer(s): 42