This could mean either of two things:
1) That the dimensions of the poster are each to be multiplied by 8/5 or
2) That the area of the poster is to be multiplied by 8/5 with the aspect ratio of 150:180 maintained.
The former interpretation is the more likely one; the latter is important when surface area is what really matters though. For example, a towel that 100cm by 50cm is four times "larger" than one that is 50cm by 25cm.
Hope this helps you.
Stephen La Rocque>