Dr. G's answer to Cheryl's Junior College 2 H2 Maths Singapore question.
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for (a), the question states that the 4 balls are taken WITH REPLACEMENT. this means that number of balls remains the same. the answer posted on left, is assuming the case of 4 balls taken without replacement.
(b)(ii) answer is incorrect due to error carried forward from (a).
answers given by EPQP in comments in main section should be the correct answers.