each student in a group of 30 students studies German,Italian,or both ,意思是：单独学德语+单独学意大利语+两门都学的学生合计30人.
studing German is three more than the total number of students studing Italian,单独学德语比单独学意大利语的学生多三人.
If the number of students that study both subjects is the same as the number of students that study exactly one subjec假如两门都学的学生数=单独学德语+单独学意大利语的学生数.
how many students in the group study only Italian?单独学意大利语的学生数为?
设：单独学意大利语的学生数为x
2X(3+x+x)=30 x=6
单独学意大利语的学生为6人