In how many hours did David cover 600 miles?
(I) David covered the first half of the distance at 40 mph.
(II) David covered the last $$\frac{1}{4}$$th of the distance at 30 mph.

if the statement (I) alone is sufficient to answer the question, but (II) does not answer the question.

if the statement {II) alone is sufficient to answer the question but (I) does not answer the question.

if both the statements (I) and (II) are sufficient to answer the question but neither statement alone is sufficient.

if both the statements (I) and (II) together are not sufficient to answer the question and additional data is required.