.
Without any theorizing, based EXCLUSIVELY on common sense, it is CLEAR and OBVIOUS
that the probability the problem asks for is .
It can not be differently.
And the condition that the family has four children is IRRELEVANT --- it is simply a kind of "white noise".
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To tutor @greenestamps
Dear @greenestamps, my experience at this forum just convinced me, that it is useless to wait an answer from the visitors,
as they do not understand the contents of their posts, at all and as a rule.
// For 90% of them, their role and functioning at this forum is "copy-paste-submit". And nothing more // (and nothing else).
Let me answer your question, instead of them.
The second phrase, which you ask about, is the description of the event,
while the last, third phrase is the real question.
Tutor @ikleyn