4,1,-3,7的24点答案：
1:4 - (1 - 3 × 7)
2:4 × 7 - 1 - 3
3:(4 × 7) - 1 - 3
4:(4 × 7 - 1) - 3
5:((4 × 7) - 1) - 3
6:4 × 7 - 3 - 1
7:(4 × 7) - 3 - 1
8:(4 × 7 - 3) - 1
9:((4 × 7) - 3) - 1
10:7 × 4 - 1 - 3
11:(7 × 4) - 1 - 3
12:(7 × 4 - 1) - 3
13:((7 × 4) - 1) - 3
14:7 × 4 - 3 - 1
15:(7 × 4) - 3 - 1
16:(7 × 4 - 3) - 1
17:((7 × 4) - 3) - 1