这里最主要要找同类项,0.31 × 258可以看作3.1 × 25.8
6.9× 25.8 + 0.31 × 258
= 6.9 × 25.8 + 3.1 × 25.8
= （6.9 +3.1）× 25.8
= 10 × 25.8
= 258