When it is in it's lowest term. Ex- 3/4.
The fraction 43/100 cannot be reduced any more!
The fraction 531/1523 cannot be reduced any more.
The fraction 17/15 cannot be reduced any more.
The fraction 39/200 cannot be reduced any more.
The fraction 47/72 cannot be reduced any more.
The fraction 77/300 cannot be reduced any more.
The fraction 7/12 cannot be reduced any more.
The fraction 141/500 cannot be reduced any more.
The fraction 47/1000 cannot be reduced any more.
The fraction 13/35 cannot be reduced any more.
The fraction 17/50 cannot be reduced any more.
The fraction 3/80 cannot be reduced any more.