Problem
The larger bottle has 8 fl oz more than the smaller. Take those 8 fl oz away from the total of 64 fl oz, leaving 64 - 8 = 56 fl oz to share as two equal smaller bottles.
Setting aside the extra makes both bottles the same size, which is easier to share.
3.OA.D.8Work BackwardsTaking the extra 8 fl oz out of the 64 fl oz total leaves 56 fl oz that stands for two equal smaller bottles.
All 64 fl oz ends up inside just the two bottles, so the total is simply the smaller bottle's oil and the larger bottle's oil added together.
Both bottles are filled to the very top, so no oil is spilled or left over and the two amounts rebuild the whole 64 fl oz with no gap.
The larger bottle's oil is the smaller bottle's oil with an extra 8 fl oz on top, so the 64 fl oz is two smaller amounts and one leftover 8.
The two capacities differ by 8, so the bigger amount splits into a part that matches the smaller amount and a separate part of 8 fl oz.
Subtracting that 8 fl oz from the total pulls back exactly the extra that was added, leaving only the two equal smaller amounts, which is 56.
Taking away 8 undoes adding 8, because subtraction reverses addition and cancels what it added.
The 56 fl oz now stands for two equal smaller bottles, so each smaller bottle holds 56 divided by 2 = 28 fl oz.
Two equal parts means cut the leftover total in half.
3.OA.A.3Work BackwardsThis only needs Grade 3 sense: take away the extra, then split the rest in half!