Question of finding the number of moles using ratios for two chemical reactions with different rates

To an acidic aqueous solution of KBrO3 (100mL, 0.1 mol/L) is added 100mL of an aqueous solution containing 0.006 moles of K2S2O3 and 0.09 moles of KI.
The following quantitative reactions occur:

Reaction (2) is instaneous, while reaction (1) is relatively slow.
By the time all the S2O3 2- anion present in solution has reacted, how many moles of BrO3- will have been consumed?

solution: BrO3- : I3- = 1:3
I3- : S2O3- = 1:2
–>BrO3- : I3-:S2O3 = 1:3:6

BUT I don’t know why I can use ratio…

hello! what is the answer in moles?

0.001 mol

Hey are you able to solve this yet?

My answer is pretty straightforward, I just think the question stem has so many unnecessary details…

(2) For S2O3 2- to completely reacted, we need .003 mol of I3-
(1) For 0.003 mol of I3- to be available, we need 0.001 mol of BrO3-

Just work backward from the equation.