(6.9-3.15)÷0.3×0.8
=6.9 ÷0.3×0.8-3.15÷0.3×0.8
=18.4-8.4
=10
2.5×(1.9+1.9+1.9+1.9)
=2.5×4×1.9
=10×1.9
=19