10.1X7.8=(10+0.1)X(8-0.2)=80+8-2-0.02=85.98
10.1X7.8=(10+0.1)×7.8=10×7.8+0.1×7.8=78+0.78=78.78
10.1*7.8=10*7.8+0.1*7.8=78+0.78=78.78
(10+0.1)*7.8=78+0.78