100÷ 2/1= 50 which is the same as 100 x ½ =50
Similarly 100 ÷ ½ is the same as 100 x 2/1 = 200
So as Tilly and sibton said, to divide by a fraction you can just flip it over and multiply by it instead.
Flipping over ⅓ gives 3/1 which equals 3.
So 90 ÷ ⅓ = 90 x 3 =270
Having read the DM's unclear question again though I think the answer is simply 3.
We were up to 90. It then said divide by a third of it (30). So 90 divide by 30 =3