Call it 21 fifths minus 13 fifths, ie 8 fifths which is one-and-three-fifths
We convert it into thirty-fifths. That gives 15 thirty-fifths minus 14 thirty-fifths which is one thirty fifth.
1 and 4 fifths.
3.4 or three and two fifths
One whole minus two fifths is three fifths.
Assuming that you mean "fifths", the answer is three fifths.
One fifth is two tenths. 8 tenths minus 2 tenths is 6 tenths or 3 fifths.