The 7 means he’ll be on the Linn side of Linn-Mathews. Linn-Mathews occupies most of the eastern face of B-J, and Linn is further south than Mathews (spelled with one T). For anyone else reading this, room numbers starting in 6 are the Matthews side. Many moons ago, Linn and Mathews were separate houses (hence the different numbers). No more.
The second number will tell you the floor. 71X is the first floor, 72X is the second. 70X would be the basement; space in housing is tight, but not that tight.