Among universities that score below 80 in 2015, what is the percentage of international students?
score below 80 refers to score < 80; in 2015 refers to year 2015; percentage of international students refers to DIVIDE(SUM(DIVIDE(MULTIPLY(num_students, pct_international_students), 100)), SUM(num_students))
SELECT SUM(CAST(T1.num_students * T1.pct_international_students AS REAL) / 100) / COUNT(*) * 100 FROM university_year AS T1 INNER JOIN university_ranking_year AS T2 ON T1.university_id = T2.university_id WHERE T2.score < 80 AND T1.year = 2015
database name:db_id university
country table country Cols: id dtype integer, country name dtype text, 
ranking_system table ranking_system Cols: id dtype integer, system name dtype text, 
ranking_criteria table ranking_criteria Cols: id dtype integer, ranking system id dtype integer, criteria name dtype text, 
university table university Cols: id dtype integer, country id dtype integer, university name dtype text, 
university_ranking_year table university_ranking_year Cols: university id dtype integer, ranking criteria id dtype integer, year dtype integer, score dtype integer, 
university_year table university_year Cols: university id dtype integer, year dtype integer, number of students dtype integer, student staff ratio dtype real, pct internation student dtype integer, pct female students dtype integer, 
