Let the bottom section has length x feet.
1. The top section is
the length of the bottom section, then the length of the top section is

2. The middle section is
the length bottom section, then the middle section has length

3. The total length of the rocket is

that is 240 feet.
Thus, you have the equation

4. Solve it. Multiply this equation by 6:

Divide it by 10:

The length of the bottom section is 144 feet, then the length of the top section is

Answer: The top section has length 24 feet