What was the question for this implicit rationale, and corresponding answer?
In the first four houses, there are a total of 4 houses * 3 gnomes = 12 gnomes. Therefore, the fifth house had 20 total gnomes – 12 gnomes = 8 gnomes.
 The answer: 8
The question: There are 5 houses on a street, and each of the first four houses has 3 gnomes in the garden. If there are a total of 20 gnomes on the street, how many gnomes does the fifth house have?