Add 3 to just the odd-index elements

I am using this help-yourself http://www.facstaff.bucknell.edu/maneval/help211/exercises.html

And have a question regarding 2. Let x = [2 5 1 6]. b. Add 3 to just the odd-index elements

They give the answer as

b = x(1:2:end) + 3

but that makes a new matrix with just two numbers in it.

Like so: "b =

    21    20
"

Is the question or the answer wrong?

Answer 1

[Azzi Abdelmalek]

x = [2 5 1 6]
x(1:2:end)= x(1:2:end) + 3

[Edward Prentice]

Many thanks.

Answer 2

[Hunter simmons]

how to make a vector z that consists of odd-indexed entries of y (y=[100:1])?