The thinking process is like this. The total length is 4/5 kilometers, which can also be divided into 8 kilometers of 10. Now we have dug 3 kilometers of 10, so we only need to dig 10 kilometers.

The calculation process is four fifths times one half minus three tenths.

We'll dig another x meters.

(3/ 10+x):4/5= 1:2

2(0.3+x)=0.8

0.6+2x=0.8

2x=0.2

x=0. 1= 1/ 10

Digging 1.00% 1 m will make the ratio of excavated length to total length 1:2.