6.6-2.91+3.4-1.09
=(6.6+3.4)-(2.91-1.09)
=10-4
=6
3.125+（6.875-1.72）-3.28
=(3.125+6.875)-(1.72+3.28 )
=10-5
=5
4.99+4.99+4.99+0.04+4.99
=4*(4.99+0.01)
=4*5
=20