What is the answer for 25 = a+8
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For first question, there is 12 marbles at first. So the probability of choosing blue marble is 3/12=1/4. Then marbles are not replaced, the total number is 11. The probability of choosing red is 5/11. So the finally answer is (1/4)*(5/11)=5/44. The same method for second question. Need to notice that you have already choose 1 white marble, so the number of white marble also changed. The answer is (4/12)*(3/11)=1/11.