You would distribute (multiply) the 5 to the 2x which would then make it 10x and you would distribute (multiply) the 5 to the 3 which would then make it 15. Because the 5 is a positive number the sign stays the same.
He would place 60 balloons in each room. First you subtract 13 from 313 and get 300. Then you divide 300 by 5 which brings you to the answer of 60 balloons per room.