(1)
(42.8-3.9)÷42.8×100%
=38.9÷42.8×100%
≈0.9089×100%
=90.89%
(2)
3.9÷(42.8-3.9)×100%
=3.9÷38.9×100%
≈0.1003×100%
=10.03%