In order to determine the length of each piece, first write the given mixed number as a fraction:
[tex]25\frac{3}{7}=\frac{25\cdot7+1\cdot3}{7}=\frac{178}{7}[/tex]Next, divide the previous result by 3:
[tex]\frac{\frac{178}{7}}{\frac{3}{1}}=\frac{178}{21}[/tex]Hence, the length of each piece is 178/21