Answer:
Given,
The number posts = 5,
Which are placing in ascending order,
Also, every post is
of the height of the next larger post.
If the larger post has the height of 3 feet,
Then the height of second larger post =

Height of third larger post =

Height of fourth larger post =

Height of fifth larger post or smallest post =

Which is the required expression,
By solving it,
The height of the smallest post =
= 1.45 feet ( approx )