I'm not sure your question has enough information to be answered properly, but I'll try to help:
Assuming each of them are sweeping 1/2 of the garge, we're gonna cut each of their times in half.
Now the girl needs 10 minutes for half the garage and her brother needs 15 minutes.
So, if they work together, the first ten minutes will overlap each other while the girls brother gets left behind to finish up for an extra five minutes.
So, when working together, the boy and the girl need 15 minutes to sweep the garage.
