42✘2.8+5.7✘28+2.8=42x2.8+5.7x10x2.8+2.8=2.8x（42+57+1）=2.8x100=280
42×2.8+5.7×28+2.8=42×2.8+5.7×10×2.8+2.8=（42+5.7×10+1）×2.8=100×2.8=280
