Consider a verticle cylinder of height H and dia D if it is filled up to 60% of its volume then the amount of fluid in it in terms of height is 0.6H if the same cylinder is tilted and kept horizontal with the same amount of fluid in it then what is the amount of liquid in it in terms of diameter D?

Consider that you show us some of your working rather than simply posting the question for us to do for you, as we Won't !

You need to find the chord that splits the area of a circle into 60:40. Like curly says, you'll need to show some working before we will give you more help.