10cm=0.1m
20cm=0.2m
设水面高x
0.4*0.3x=0.4*0.3*0.1+0.2*0.2x
0.12x=0.012+0.04x
0.08x=0.012
x=0.012÷0.08
x=0.15
答 水面高0.15米