Answer:
you enter x 1/3 and will get the answer.
You haven't given the dimensions so an example is below.
Explanation:
Ex 1
Poster current size 60inches
60 x 1/3 = 20 = 20inches
Ex 2
Poster current size 63 inches
60 x 1/3 = 21 = 21inches
Ex 3
Poster current size 67 inches
67 x 1/3 = 22.33 = 22.33 inches
Ex 4
Poster current size 81 x 47 inches
81 x 1/3 = 27 47 x 1/3 = 15.66
This means the poster at 1/3 size is 27in x 15.66in