You can doublecheck the solution if you plug the value 'r = 13/4' or written as a decimal 'r=3.25' into:

0.5(r + 2.75) = 3

0.5(3.25 + 2.75) = 3

0.5*6 = 3

3 = 3 thus r = 13/4 (or r = 3.25) is indeed the correct solution.