13. There are 9 people lost in the mountains, and the food they have is only enough to last for 5 days. One day later, these 9 people encounter another group of lost people who have no food at all. After calculation, if the two groups share the food, with each person eating the same amount of food per day, the food will only last them for another 3 days. How many people are in the second group of lost people?
【Analysis】Let's assume each person eats 1 portion of food per day, then the original amount of food is 45 portions. After 1 day, 36 portions remain. If this can last for 3 days, then each day they eat $36 \div 3=12$ portions, meaning there are 12 people at this time, so the number of people from the second team who got lost is 3.