A spy tries to infiltrate into a Spanish base. In order to do so, he pinpointed a spot where he could listen the people outside of the gate who were asked to answer a short question. These questions were:
First outsider answer: 7.
Second outsider answer: 9.
Once the spy heard this, he realised that he always needed to give the half of the number as a response. So he went to the base,
Spy answer: 3.
The spy was immediately killed.
Why this happened?