In a village, there are 10 houses in a row. Every day, one person from each house goes to the market to buy groceries. On Monday, they all go at different times of the day. If John from House 1 and Mary from House 2 always go together, what is the probability that on Tuesday, they will meet again at the market?
The probability is 1/9.

Here's the reasoning:

1. There are 10 people going to the market, but since John and Mary always go together, we can consider them as a single entity.
2. So now, we have 9 entities going to the market (8 individuals plus the combined John and Mary).
3. Since they all go at different times, there are 9! (9 factorial) ways for these entities to arrange their visits to the market.
4. Now let's count the number of arrangements where John and Mary meet again on Tuesday.
5. Since John and Mary always go together, we can think of them as occupying a single time slot.
6. There are 9 available time slots for the other 8 individuals, so there are 8! ways for the others to arrange their visits around John and Mary's time slot.
7. Therefore, the probability of John and Mary meeting again on Tuesday is the ratio of favorable outcomes (8!) to total possible outcomes (9!), which simplifies to 1/9.