Assuming that the crude oil in barrels A, B, C and D is a, B, C and D liters respectively, the following equation can be listed according to the meaning of the question:

a+ 1/2b=30， 1/2b+ 1/3c=30，2/3c+ 1/4d=30，3/4d=30

So we can solve a= 10, b=40, c=30 and d=40.