The given data of the problem are 42 combined dvds from Carly and Rob and Carly having 4 more dvds than Rob. Now, the problem wants to find out how many dvds did Rob and Carly owned before combining them. We got a total of 42 dvds at hand, so let's divide it by 2 for Carly and Rob so we have 21 dvds each however, the problem emphasized that Carly had 4 more dvds than Rob. So, 21 dvds added by 4 dvds is the total number of dvds that Carly previously owned meaning Carly owned 25 dvds before while Rob owned only 17 dvds.
