Explanation:
remember, that a/a = 1, and this is always the "whole" in such fraction scenarios.
in our case here : 9/9 or 3/3.
4 books for 2/9 of his money.
8 books for then 2× 2/9 = 4/9 of his money.
that is together
12 books for 6/9 = 2/3 of his money.
that leaves 3/9 = 1/3 of his money for the 2 identical bags.
so, 1/3 of his money for 2 bags.
how many bags for the whole (3/3) money ?
1/3 × x = 3/3
x = 3
he could have bought 3×2 = 6 bags with the original sum of money.