probability – question of algebra Bayes theorem

A protein that has the objective of helping the growth of pigs is administered to pigs at their lunch.
One researcher observes that few pigs may have developed tumors as a result of their feeding, which comprises the protein.
The researcher who added the protein discovers that for a random pig that acquires tumors, there is a probability that goes from p = 1/10, when a pig has a standard lunch
a p = (1/2) when given the lunch that has the protein present.
The research of the researchers has 100 pigs, where the number of pigs that receive lunch with the present protein is unknown (I refer to this as n).

My question is that:
A second researcher chooses at random a pig from the cohort of 100 pigs. If the pig has tumors, find the probability that the pig has received the lunch where the protein is present.