shyind wrote:

A straight fence is to be constructed from posts 6 inches wide and separated by lengths of chain 5 feet long. If a certain fence begins and ends with a post, which of the following could not be the length of the fence in feet? (12 inches = 1 foot)

A. 17

B. 28

C. 35

D. 39

E. 50

The fence begins and ends with a post: {post = 0.5 feet}{chains = 5 feet}{post = 0.5 feet}...{chains = 5 feet}{post = 0.5 feet}

The minimum length is with 2 posts = 0.5 + 5 + 0.5 = 6 feet. To increase the fence, each time we should add{post = 0.5 feet}{chains = 5 feet}, so 5.5 feet. Thus, the lengths could be:

6 feet;

6 + 5.5 = 11.5 feet;

11.5 + 5.5 = 17 feet;

17 + 5.5 = 22.5 feet;

22.5 + 5.5 = 28 feet;

28 + 5.5 = 33.5 feet;

33.5 + 5.5 = 39 feet.

...

We skipped 35 feet.

Answer: C.

Or: options A, B, D and E are in A.P. 17, 28, 39, 50 (common difference 11). 35 clearly doesn't belong to this pattern.