sara is working on making her house more eco-friendly. she has a number of upgrades she wants to make such as replacing her toilet that uses 5 gallons per flush
with a new toilet that uses 1.2 gallons per flush. she also wants to replace a leaky faucet in her kitchen. finally, she would like to replace her old washing machine that
uses 35 gallons of water per load with a new high-efficiency model that only uses 10 gallons per load.

which item should sara replace first and why? support your answer with evidence and reasoning.

Toilet flush

Explanation:

In order to decide which component is to be removed first, Sara must evaluate the maximum loss of resources and energy from all the devices and equipment used in house.  

Washing machine uses the maximum amount of water  i.e 35 gallons and replacing it with a high end washing machine will save 25 gallons of water. However, changing the toilet flush will save nearly 3.8 gallons of water per flush. Assuming that on an average toilet is flushed 10 time a day, then the total saving of water by changing flush would be 3.8 * 10 i.e 38 gallons.  

Hence, Sara must first change the toilet flush.

substitution is the answer in this case