The fare to the Boston University campus should be
By subway: The airport is connected to Boston University via the subway (called the "T"). Boston University lies on the Green Line, sandwiched between the B line to the north and C line to the south. The easiest access to BU from the airport is via the B line.
At Logan, get on the Silver Line bus towards South Station. At South Station, get on the Red Line towards Alewife, you will transfer at Park Street.
At Park Street, go upstairs to the Green Line trolley platforms and take the "B" line towards Boston College.
The Department of Mathematics and Statistics is in the MCS building, one block south of the "BU East" stop on the Green Line on Cummington Street.