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Mark has a bag that contains exactly 3 black marbles, 6 gold marbles, purple marbles and 6 red marbles. Mark finds a number of white marbles and adds them to the bag. He tells Sam that if he draws a marble at random from the bag, the probability of being black or gold is 3/7. How many white marbles did Mark add into the bag?

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SmartyIsMe
Assuming there are 6 purple marbles, there would be 0 white marbles added. If there are not 6 purple marbles, please let me know so I can help.
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cedarwood79
Okie doke. So, to find out the amount of marbles in the bag so far, we can add the amounts of each color together. 3 + 6 + 2 + 6 is 17. There are 17 marbles in the bag, but we add white marbles in the bag, and the chances of getting a gold or black marble is 3/7. 3 + 6 is 9. And if know about multiples, the LCM of 3 and 7 is 21. 21 - 17 is 4. Let's set up a proportion to check this: 3/7 = x/21. Cross multiply values to get 63 = 7x. Divide each side by 7 to get 9. And that satisfies the problem. Mark added 4 white marbles in the bag.

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