Add 6oz of chemical B and remove 1/3 of the total mixture.
I might always be wrong.
Step-by-step explanation:
Since 8x3=24 and since 12x3=36 you need to add 6oz of chemical B to have 24oz of chemical A and 36oz of chemical B.
Take 1/3 of the total mixture to have 8oz of chemical A and 12oz of chemical B.