19.4×6.1×2.3+7.7=20x6.1x2.3-0.6x6.1x2.3+7.7=122x2+122x0.3-3.66x2-3.66x0.3+7.7=244+36.6-7.32-1.098+7.7=280.6+7.7-8.418=280-(8.418-8.3)=280-0.118=279.882
（3/7+1/7）+1/6-3/7=1/7+1/6 =（6+7）/(6*7) =13/42
see
