分母減去?$1$?后可約分為?$\frac {1}{6},$?
?$\frac {1}{6}$?約分前可?$ $?能是?$\frac {2}{12}、$??$\frac {3}{18}、$??$\frac {4}{24}、$??$\frac {5}{30}……$?
故原分數(shù)可能是?$\frac {2}{13}、$??$\frac {3}{19}、$??$\frac {4}{25}、$??$\frac {5}{31}……$?
其中只有?$\frac {4}{25}$?的分子加上?$1$?后可以約分為?$\frac {1}{5},$?
所以這個分數(shù)是?$\frac {4}{25}。$?