3958. Electric bulbs are manufactured in two factories, the first producing 60% of the total, the second 40%. The production of the first plant contains 70% of the lamps of the highest grade, the second - 80%. The store receives products from two factories. The purchased lamp was not the highest grade. Find the probability that this lamp is made at the first factory.

Detailed solution. Decorated in Microsoft Word 2003 (Quest decided to use the formula editor)