if debt-to-total assets ratio is 40%. What is debt-to-equity ratio?

I don't think you've given enough information to determine the answer. Check this site.

http://www.investopedia.com/terms/d/debtequityratio.asp

Thats all the question states

To calculate the debt-to-equity ratio, we need to know the values of both total debt and total equity. The debt-to-equity ratio measures the proportion of a company's debt to its equity, indicating how much financing is provided by creditors compared to shareholders.

The debt-to-total assets ratio alone does not provide sufficient information to directly calculate the debt-to-equity ratio. It only gives a measure of the proportion of a company's assets financed by debt.

To find the debt-to-equity ratio, you need to have information on the total debt and total equity of the company. The total debt is the sum of all the company's long-term and short-term debt. Total equity refers to the difference between the company's total assets and total liabilities.

Once you have the total debt and total equity values, you can calculate the debt-to-equity ratio using the following formula:

Debt-to-Equity Ratio = Total Debt / Total Equity

Without the specific values of total debt and total equity, it is not possible to determine the debt-to-equity ratio in this case.