100×0.83+83×99 =100×83/100+83×（100-1）=8300
4.7×63+3.7×47 =4.7×63+37×4.7=4.7×（63+37）=470
3.6x + 2.4 x =0.36 ,6x=0.36,x=0.06
5（x-3.1）=15.5,5x-15.5=15.5,5x=31,x=6.2