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 13/40 cannot be reduced any more.
The fraction 19/21 cannot be reduced any more.
The fraction 22/13 cannot be reduced any more.
The fraction 4/15 cannot be reduced any more.
The fraction 119/1000 cannot be reduced any more.
The fraction 2083/1000 cannot be reduced any more.
The fraction 1/25 cannot be reduced any more.
The fraction 77/6 cannot be reduced any more.