In the hypothetical in which they do consider the army possibility, the parents would have decided that George should not pay the $1200 if Richard was in the army.
This result illustrates the Hypothetical Intent Test. Under that test,
(1) you determine the main purpose of the agreement, and
(2) determine what the parties would have reasonably agreed to if they had considered the unforeseen possibility which raises the question of what the parties obligations are.
The answer determines the parties' hypothetical intent, the intent they would have had if they had taken the unforeseen possibility into account.