Mark and Simon had 166 notebooks together. Mark had 34 more notebooks than Simon. Both of them sold some of their notebooks. Simon sold 30% less notebooks than what Mark sold. In the end, Simon had half as many notebooks left as Mark. How many notebooks had Mark left?

	Mark (1)	Simon (2)	Make p the same (2)x2 = (3)
Comparing Mark and Simon at first	34 more
Before	100	66	132
Change	- 10 u	- 7 u	- 14 u
After	2 p	1 p	2 p

Number of notebooks that Simon had at first
= (166 - 34) ÷ 2
= 132 ÷ 2
= 66

Number of notebooks that Mark had at first
= 66 + 34
= 100

Number of notebooks that Simon sold in percent
= 100% - 30%
= 70%

70% = ⁷⁰₁₀₀ = ⁷₁₀

Make p the same.

(1) = (3)
100 - 10 u = 132 - 14 u
14 u - 10 u = 132 - 100
4 u = 32

1 u = 32 ÷ 4 = 8

Number of notebooks that Mark had left
= 100 - 10 u
= 100 - 10 x 8
= 100 - 80
= 20

Answer(s): 20