Let's use the information given in the problem to set up an equation to model the situation.
Juan starts with 105 necklaces and his cousin gives him R necklaces, so the total number of necklaces after his cousin gives him some is:
105 + R
After his cousin gives him R necklaces, Juan has a total of 131 necklaces. So we can write:
105 + R = 131
This equation says that the number of necklaces Juan starts with (105) plus the number of necklaces his cousin gives him (R) equals the total number of necklaces he has after his cousin gives him some (131).
Therefore, the equation that correctly models this situation is:
105 + R = 131