cliff collects marbles. 1/2 of his collection is blue. of the blue marbles 1/2 of these are a dark blue. how much of cliff's marbles collection is dark blue.

25%

Step-by-step explanation:

50% of marbles are blue.

Half of 50% is 25%.

For example:

You have 100 marbles. Half are blue, so fifty marbles are blue. Half of those 50 marbles are dark blue, so 50:2 is 25. 25 is 1/4 of 100, which is also 25%.