67.8×3.2+6.8×67.8=67.8×（3.2+6.8）=67.8x10=678
67.8×3.2+6.8×67.8= 67.8×(3.2+6.8)= 67.8×10= 678
67.8x3.2+6.8x67.8=67.8X(3.2+6.8) 乘法对于加法的结合律=67.8X10=678
