3,13,12,13的24点答案：
1:(3 - 13 ÷ 13) × 12 =24
2:(3 - (13 ÷ 13)) × 12=24
3:12 × (3 - 13 ÷ 13)=24
4:12 × (3 - (13 ÷ 13))=24