SELECT AVG(T2.gradepoint) FROM ENROLLED_IN AS T1 JOIN GRADECONVERSION AS T2 JOIN STUDENT AS T3 ON T1.Grade = T2.lettergrade AND T1.StuID = T3.StuID WHERE T3.LName = "Smith"
Find the average grade point of student whose last name is Smith.
CREATE TABLE ENROLLED_IN (
    Grade VARCHAR,
    StuID VARCHAR
)

CREATE TABLE STUDENT (
    LName VARCHAR,
    StuID VARCHAR
)

CREATE TABLE GRADECONVERSION (
    gradepoint INTEGER,
    lettergrade VARCHAR
)