Comparative Sentences (比较句：A没有B) - Is the grammar of the following combined sentence correct?

Question

This question is about Comparative Sentences (比较句：A没有B)

So we have the following two sentences:

• 爸爸的手机2000 块。
• 哥哥的手机6000 块。

The assignment is to make these into one sentence using the A没有B structure and the character 贵.

Is the following answer correct? Or is there something wrong with this sentence please:

爸爸的手机没有哥哥贵。

Or should you repeat 的 at least? ->

爸爸的手机没有哥哥的贵。

Or should you always repeat the structure from the first half completely? ->

爸爸的手机没有哥哥的手机贵。

?

Answer 1

The comparison is between [the father's cell phone] and [the brother's cell phone]

爸爸的手机没有哥哥贵 would mean "my father's cell phone is not as expensive as my brother"

Either "爸爸的手机没有哥哥的手机贵" or "爸爸的手机没有哥哥的贵" would be correct