Final answer:
Mimi can pour an additional 6 full buckets of water into the pool.
Step-by-step explanation:
To solve for the number of additional full buckets of water Mimi can pour into the pool, we need to subtract the amount of water already poured from the pool's capacity and divide by the capacity of each bucket.
The amount of water already poured into the pool is 12 buckets * 7.5 gallons per bucket = 90 gallons.
The remaining capacity of the pool is 138 gallons - 90 gallons = 48 gallons.
Therefore, Mimi can pour an additional n buckets, where n = 48 gallons / 7.5 gallons per bucket ≈ 6.4 buckets.
Since she can't pour a fraction of a bucket, she can pour an additional 6 full buckets of water into the pool.