(单词翻译:单击)
每日一题
Mathematics>Standard Multiple Choice
Read the following SAT test question and then click on a button to select your answer.
A jar contains only red marbles and green marbles. If a marble is selected at random from the jar, the probability that a red marble will be selected is 2/3. If there are 36 green marbles in the jar, how many red marbles are there in the jar?
(A) 18
(B) 24
(C) 54
(D) 72
(E) 108
答案和解析
答案:D
解析:
Choice (D) is correct. Let r be the number of red marbles in the jar. The probability of selecting a red marble is equal to the number of red marbles divided by the total number of marbles. Since the probability of selecting a red marble is 2/3.There are 36 green marbles and r red marbles, so the previous equation becomes 2/3=r/r+36. This simplifies to 2(r+36)=3r, or 2r+72=3r. Therefore, the number of red marbles, r, is 72.