One clerk works out 24% of 350 and another works out 35% of 240. Which of the two figures is larger?

24% of 350 = 84 and 35% of 240 = 84, so the two are equal. This is not a coincidence: a% of $b$ = (a $x$ b)/$100=b$% of a, so swapping the rate and the quantity never changes the answer. It makes a handy shortcut too - 48% of 25 looks awkward, but it is the same thing as 25% of 48, which is 12. The two 'larger' options assume that the bigger rate, or the bigger quantity, must win; only the product of the two matters. Nothing about units is missing here.