Final answer:
The Johnsons were using 11.5 tanks of gas per day.
Step-by-step explanation:
To find out how many tanks of gas the Johnsons were using per day, we need to divide the total tanks used by the number of days. Based on the information given, the Johnsons used 23 tanks in 2 days. So, the tanks of gas used per day can be calculated as:
23 tanks / 2 days = 11.5 tanks of gas per day