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.
The chance of the 1st selection greater than 3 is 6/10 or 3/5
The chance of the 2nd selection greater than 3 is 5/9
The chance of the 3rd selection greater than 3 is 4/8 or 1/2
The chance of the 4th selection greater than 3 is 3/7
The chance of 5th selection greater than 3 is 2/6 or 1/3
Multiply all chances together we get: 1/42
So, the answer is 1/42 chance the 5 numbers selected are greater than 3