Answer:
4 cm
Explanation:
The milk fills 12 cm of the 15 cm height, so fills 12/15 = 4/5 of the volume of the container.
When the container is laid flat so that the 5 cm direction is the height, the milk still fills 4/5 of the height. The dept of the milk in that configuration is ...
(4/5)(5 cm) = 4 cm