sat 数学题一道~

sat 数学题一道~
studing German is three morethan the total number of students studing Italian,If the number of students that study both subjects is the same as the number of students that study exactly one subjec,how many students in the group study only Italian?
不好意思啊，题目有错而且每翻译，是这样的
有一群30人的学生学习了德语，意大利语，或是两者都学了。所有学习德语的学生人数比学习意大利语的人数多了3个，如果学习两种语言的学生人数和只学习一门语言的学生人数一样，求多少学生学习了意大利语？