The first urn contains 4 black and 6 white balls. The second urn contains 4 white and 6 black balls.

The first urn contains 4 black and 6 white balls. The second urn contains 4 white and 6 black balls. In the third urn there are 3 white and 7 black balls. From a randomly taken urn, one ball was taken out, which turned out to be white. Then the probability that this ball was removed from the first urn is equal to …

This is the probability of the sum of two incompatible events.
1 – The 1st urn is selected and a white ball is taken out;
2 – The 2nd urn is selected and a white ball is taken out;
P1 = 0.5 • 0.2 = 0.1; P2 = 0.5 * 0.4 = 0.2.
Then the required probability
P = P1 + P2 = 0.1 + 0.2 = 0.3.