Suppose that a homeowner notices a 20 percent increase in the water bill for July. The homeowner traces this increase to four sources: a running toilet, a dripping faucet, a guest who visited for two days, and a broken sprinkler head. Further study shows that the broken toilet accounts for 8 percent of the increase, the faucet 2 percent, and the visiting guest 4 percent. The homeowner concludes that the remaining 8 percent is attributable to the broken sprinkler head. What method did the homeowner use in drawing this conclusion