Answer: The sash will have 20 diamonds when it is completed.
Explanation:
Given: The length of American sash= 8 feet
Since, Joseph wants to add a diamond shape every 0.4 ft along the sash.
Therefore, the number of diamonds he will need to complete the sash=

⇒ The number of diamonds he will need to complete the sash=

Hence, the number of diamonds he will need to complete the sash= 20