set all of his money as x.
3/5x is 4 highlighters, so 3/10x is 2 highlighters.
he only spent 1/10x of his money buying 10 rulers, which answers the first question given.
(b) he spends all his money buying rulers. which means ignoring the free rulers, he could buy 100 rulers. but then, since there are 8*12 = 96, we know he gets an extra 8 rulers and hence have 108 rulers if he spent all his money at first on rulers.