2015 AMC 8 Problems/Problem 7
Each of two boxes contains three chips numbered , , . A chip is drawn randomly from each box and the numbers on the two chips are multiplied. What is the probability that their product is even?
We can instead calculate the probability that their product is odd, and subtract this from . In order to get an odd product, we have to draw an odd number from each box. We have a probability of drawing an odd number from one box, so there is a probability of having an odd product. Thus, there is a probability of having an even product.
You can also make this problem into a spinner problem. You have the first spinner with equally divided
sections, and You make a second spinner that is identical to the first, with equal sections of
,, and . If the first spinner lands on , to be even, it must land on two. You write down the first
combination of numbers . Next, if the spinner lands on , it can land on any number on the second
spinner. We now have the combinations of . Finally, if the first spinner ends on , we
have Since there are possible combinations, and we have evens, the final answer is
We can also list out the numbers. Hat A has chips , , and , and Hat B also has chips , , and . Chip (from Hat A)
could be with 3 partners from Hat B. This is also the same for chips and from Hat A. total sums. Chip could be multiplied with 2 other chips to make an even product, just like chip . Chip can only multiply with 1 chip. . The answer is .
Here is another way:
Let's start by finding the denominator: Total choices. There are chips we can choose from in the 1st box, and chips we can choose from in the 2nd box. We do , and get . Now - to find the numerator: Desired choices. To get an even number, we need to pick 2 from at least one of the boxes. There are choices as to finding which box we will draw the 2 from. Then we have choices from the other box to pick any of the other chips, .
However, we are over counting, the configuration twice, and so we subtract that one configuration from our total. . Thus, our answer is .
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