Just looking at the portion circled in orange, we know that he spent 1/2 of the remaining money and an additional $20 on the stationary. This means that what is left is the other 1/2 minus the $20. Since the question told us that he was left with $32 in the end, this means that this 1/2 is ($32 + $20 = $52).
The total amount he had left after buying the buying the books is $52 x 2 = $104.
So the amount he spent on stationary is $104 – $32 = $72.