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By the definition of the CONDITIONAL PROBABILITY,
P(C|D) = P(C AND D) / P(D).
So, we have 0.6 = P(C AND D) / 0.5.
It implies P(C AND D) = 0.6*0.5 = 0.3. ANSWER
Solved.
Notice that this info " Let P(C) = 0.4 " is excessive; it was not used in the solution, and therefore is IRRELEVANT to the problem.