解:設(shè)甲電線?$x$?米。
?$x×({1-\frac {1} {3}} )+ ({2000-x} )×({1-\frac {1} {4}} )=1400$?
? $x×\frac {2} {3}+ ({2000-x} )×\frac {3} {4}=1400$?
? $\frac {2} {3}x+1500-\frac {3} {4}x=1400$?
? $\frac {3} {4}x-\frac {2} {3}x=1500-1400$?
? $\frac {1} {12}x=100$?
? $x=100\div \frac {1} {12}$?
? $x=1200$?
? $2000-1200=800 ({米} )$?
答:甲電線?$1200$?米,乙電線?$800$?米?$.$?