This couple were traveling in an overnight train and were in a cabin with three sleeping beds, top, middle and lower.    The reservation was all screwed up and a teenage kid happen to get the middle bed.    The horny couple worked their way out.    The husband would come down and have sex and go back to his top bed.    The kid was awake and noticed all this and at dead of night heard the husband fast asleep and snoring.    The lights were out and he goes down and fucks the woman.      Two hours later, goes down again and they have sex.

In the morning the husband and wife talk and the guy says I fucked you three times.   The wife says no you fucked me 5 times.     Husband was totally lost and the kid in the middle just enjoyed the math!