198乘以0.51用简便计算怎样做

198 * 0.51 =200 * 0.51-2 * 0.51 =102-1.02 =100.98
40.7-2.6 *9.3
=40.7 -2.6*（10 -0.7）
=40.7 -26 +2.6*0.7
= 14.7 +2.6*0.7
=14.7 +2.6*（1 -0.3）
=14.7 +2.6 -0.78
=17.3-0.78
=16.52
5.2*10.1=5.2*（10 +0.1）=52 +0.52=52.52