has done is work out that if one draws from a group of five rather than six, that getting a result of 2/5 is very easy. take away a black ball and we have one one green and one black = 2/5.
now how to get rid of one pesky black ball? easy, make sure it is already removed with the phrase “given the first ball drawn was black”.
there’s no need for complicated conditional thinking at all. the person has used the lowest possible level of thinking about probability.
i’m not necessarily so convinced. the result of the question needed to be 2/5 so why not use one of those branches instead 