?$[$?跟蹤練習(xí)?$ 2]$?
?$\frac {1}{7}×5 = \frac {5}{7}$?
?$1 - \frac {5}{7} = \frac {2}{7}$?
?$\frac {2}{7}÷6 = \frac {1}{21}$?
?$1 - \frac {1}{5} = \frac {4}{5}$?
?$\frac {1}{21}÷\frac {4}{5} = \frac {5}{84}$?
?$\frac {1}{7} - \frac {5}{84} = \frac {1}{12}$?
?$1÷\frac {1}{12} = 12(天)$
答：甲工程隊單獨(dú)完成這項工程要12天。?
