Final answer:
Sara should replace her toilet first due to the high frequency of usage and substantial reduction in water usage per flush, offering the greatest overall water savings in her home.
Step-by-step explanation:
Sara should consider the frequency of use and the amount of water saved per use when deciding which item to replace first in her quest to make her house more eco-friendly. The toilet is likely used multiple times daily, and upgrading from a unit that uses 5 gallons per flush to one that uses 1.2 gallons will have a significant impact by saving 3.8 gallons per flush. Given that toilet flushing is the largest indoor use of water in households, this could result in the greatest overall water savings.
Even though the washing machine uses more water per load than the old toilet does per flush, the frequency of use is typically less than that of the toilet. Meanwhile, fixing the leaky faucet is also important, as leaks can waste a substantial amount of water over time. In conclusion, based on water savings potential and usage frequency, the toilet replacement should be the highest priority.