10th question in question and answer, chp 8

[30binanfd]

The 10th question of chapter 8 question and answer asks what is the best insulator. It says how the best insulator has the highest R value and R is calculated by dividing thickness by thermal conductivity, k. Dividing 75 in of thick fiberglass by k= 0.05 I get 1500 but apparently the answer is 0.7 in. thick rock wool with k= 0.04 but this only gives a relative R of 17.5. What is going on with these questions? This isn't anywhere in the textbook and the passage doesn't give enough info to answer the 7th or 8th questions. Thanks for your time.