For these types of word problems, what I like to do is read the question first, and then go to the beginning.
We want to know how many more notebooks Paul can buy with 1/2 of his remaining money, and since he's already spent 1/4 of his money, that means we have to take half of 3/4, which is 3/8. So the question is asking, how many notebooks can Paul buy with 3/8 of his original money.
Now, let's set some variables.
M = Paul's total amount of money
T = Cost of each thumb drive
2/3T = Cost of each notebook (since the cost of each notebook is 2/3 the cost of each thumb drive)
Paul spent 1/4M on 6 notebooks and 4 thumb drives, so
6 * (2/3T) + 4 * T = 1/4M
Simplifying the left side gives us:
4T+4T=1/4M
8T=1/4M
And now, solve for M:
M=32T
Since we want to know how many notebooks Paul can buy with 3/8M, not M, we multiply 32T by 3/8 to get 12T.
Finally, to get the answer, all we need to do is divide how much money Paul wants to spend by the cost of each notebook to see how many notebooks he can buy.
12T/(2/3T) = 18
Final answer: Paul can buy 18 more notebooks with 1/2 of his remaining money.
Sorry for the lengthy response, hope this helps!