简便计算7.4+7.8-2.8+12.6=（7.4+12.6）+（7.8-2.8）=20+5=25
原式=7+0.4+7+0.8-28/126=14+1.2-14/63=15.2-12.6/63-1.4/63=15.2-0.2-0.02-0.14/63≈14.98
