设该生在面试时的得分为X,由题设条件知X的可能取值为-15,0,15,30,
P(X=-15)=
C 03 (
1
3 ) 3 =
1
27 ,
P(X=0)=
C 13 (
1
3 ) 2 (
2
3 ) =
2
9 ,
P(X=15)=
C 23 (
1
3 )(
2
3 ) 2 =
4
9 ,
P(X=30)=
C 33 (
2
3 ) 3 =
8
27 ,
∴EX=-15×
1
27 +0×
2
9 +15×
4
9 +30×
8
27 =15.
∴该学生在面试时得分的期望值为15分.
故答案为:15.