In a book shop, the number of rulers was ⁵₁₂ of the total number of rulers and correction tapes together. Tommy sold the rulers and correction tapes in sets of 2 rulers and 10 correction tapes. After selling all the correction tapes, he still had 396 rulers left. How many correction tapes were in the book shop were sold?

	Rulers	Correction Tapes
Before	5x10 = 50 u	7x10 = 70 u
Change	- 2x7 =  - 14 u	- 10x7 = - 70 u
After	396	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 7 and 10 is 70.

Number of rulers sold
= 50 u - 14 u
= 36 u

36 u = 396

1 u = 396 ÷ 36 = 11

Number of correction tapes were sold
= 70 u
= 70 x 11
= 770

Answer(s): 770