If I do not have purchase information for mutual fund shares sold, can I use average per share basis to determine cost?

Question

How do I calculate the average basis for the sale of mutual fund shares?

Answer

In order to figure your gain or loss using an average basis, you must have acquired the shares at various times and prices.

To calculate average basis:

• Add up the cost of all the shares you own in the mutual fund.
• Divide that result by the total number of shares you own. This gives you your average per share.
• Multiply the average per share by the number of shares sold.

Source:  https://www.irs.gov/faqs/capital-gains-losses-and-sale-of-home/mutual-funds-costs-distributions-etc/...