I will post the question and explanation from the book as it is and then ask the question. Here it goes:

By what percent did the median household income in Country Y decrease from 1970 to 1980?

(1) In 1970 the median household income in country Y was 2/3 of the median household income in Country X

(2) In 1980 the median household income in Country Y was 1/2 of the median household income in Country X.

The books lists answer E and gives this explanation:

In order to answer this question, it is neceassary to have the data on the median household income in Country Y for both 1970 and 1980.

(1) This offers no information about the actual income in Country X, so the 1970 data for Country Y cannot be determined, and there are no data for Country Y for 1980; NOT sufficient.

(2) This provides no facts about the income of Country X in 1980, and there are no data for Country Y for 1970; NOT sufficient.

Both (1) and (2) together compare the median household income of Country Y to that of Country X for the pertinent dates. However, without any concrete data about Country X, the comparisons are not helpful in calculating the percent decrease in Country Y from 1970 to 1980.

The correct answer is E.

Wouldn't the answer be C? Please check my thinking and comment:

Statement (1) suggests that Y = 66.6X and statement (2) suggests that Y = 0.5X.

Thus, could we say that 66.6X - 0.5X would give us 0.04, which would stand for the percent decrease?

Comments appreciated!