A ballpoint pen cost 8 times as much as an exercise book. John spent 2/5 of his money one exercise books and 1/3 of the remainder on 3 ballpoint pens. How many exercise books did he buy?

Answer 1

1/3 (1 - 2/5) = 1/5 spent on 3 books
so each book cost 1/15 of his money
each pen cost 8/15 of his money
he could only buy one exercise book.

Answer 2

If your question means:

John spent 2 / 5 of his money on exercise books, not for ​one exercise book

then:

m = John´s money

b = price of ballpoint pen

e = price of an exercise book

b = 8 e

When John spent 2 m / 5 on exercise books remainder = m - 2 m / 5 = 5 m / 5 - 2 m / 5 = 3 m / 5

1 / 3 of the remainder = ( 1 / 3 ) ∙ 3 m / 5 = m / 5

1 / 3 of the remainder = 3 ballpoint pens

m / 5 = 3 b

Multiply both sides by 5

m = 15 b

m = 15 ∙ 8 e

m = 120 e

For his money he can buy 120 exercise books.

John spent 2 / 5 of his money on exercise books means:

2 m / 5 = 2 / 5 ∙120 e = 240 e / 5 = 48 e

He buy 48 exercise books.

Answer 3

my bad - I got lost in the details.
Thanks for watching.