4/55x56
=4/55x(55+1)
=4/55x55+4/55x1
=4+4/55
=4又55分之4

79x10.1
=79x(10+0.1)
=79x10+79x0.1
=790+7.9
=797.9

4.3-2.82+6.7-6.12
=(4.3+6.7)-(2.82+6.12)
=11-8.94
=2.06