1. Find how many of the board can be used for picture frames:
[tex]10\dfrac{1}{2}-2\dfrac{3}{8}=\dfrac{21}{2}-\dfrac{19}{8}=\dfrac{21\cdot 4-19}{8}=\dfrac{65}{8}\ ft.[/tex]
2. Each picture frame needs [tex]1\dfrac{2}{3}=\dfrac{5}{3}[/tex] feet of the board, then the farmer can complete
[tex]\dfrac{65}{8}:\dfrac{5}{3}=\dfrac{65}{8}\cdot \dfrac{3}{5}=\dfrac{39}{8}=4\dfrac{7}{8}[/tex]
picture frames.
Answer: at most he can complete 4 picture frames.