Answer:
54.6 inches
Explanation:
Let x be the height of Duc's brother.
If Duc is 39 inches tall, and he is 5/7 as tall as his brother, then we can express this relationship as:

To find the height of Duc's brother, solve the equation for x.
Multiply both sides of the equation by 7:


Divide both sides of the equation by 5:


Therefore, the height of Duc's brother is 54.6 inches.