Maths Olympiad Prep

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Combinatorics Difficulty 3.6 AMC 10/12 Find the answer

Each of two boxes contains three chips numbered 11, 22, 33. 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?

Pick one

Solution

Solution 1.1
(This solution is similar to Solution 2.)
Let's make this a problem with boxes.
In total, there are 9 products derived from these numbers (because 3 numbers per box). This would be our denominator.
Every time a spinner lands on 2, we get an even product. These are (1,2)(1,2), (2,1)(2,1), (2,2)(2,2), (2,3)(2,3), and finally (3,2)(3,2).
Going back, we see there are 33=93\cdot3=9 possible combinations, and we have 55 evens, the final answer is
(E) 59\boxed{\textbf{(E) }\frac{5}{9}}.

Solution 1
You can also make this problem into a spinner problem. You have the first spinner with 33 equally divided
sections: 1,21, 2, and 33. You make a second spinner that is identical to the first, with 33 equal sections of
11,22, and 33. If the first spinner lands on 11, it must land on two for the result to be even. You write down the first
combination of numbers: (1,2)(1,2). Next, if the spinner lands on 22, it can land on any number on the second
spinner. We now have the combinations of (1,2),(2,1),(2,2),(1,2) ,(2,1), (2,2), and (2,3)(2,3). Finally, if the first spinner ends on 33, we
have (3,2).(3,2). Since there are 33=93\cdot3=9 possible combinations, and we have 55 evens, the final answer is
(E) 59\boxed{\textbf{(E) }\frac{5}{9}}.

Solution 3
We can also list out the numbers. Box A has chips 11, 22, and 33, and Box B also has chips 11, 22, and 33. Chip 11 (from Box A)
could be with 3 partners from Box B. This is also the same for chips 22 and 33 from Box A. 3+3+3=93+3+3=9 total sums. Chip 11 could be
multiplied with 2 other chips to make an even product, just like chip 33. Chip 22 can only multiply with 1 chip. 2+2+1=52+2+1=5. The answer is (E) 59\boxed{\textbf{(E) }\frac{5}{9}}.

Solution 4
Here is another way:
Let's start by finding the denominator: Total choices.
There are 33 chips we can choose from in the 1st box, and 33 chips we can choose from in the 2nd box. We do 333*3, and get 99.
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 22 choices as to finding which box we will draw the 2 from. Then, we have 33 choices from the other box to pick any of the other chips, 1,2,1, 2, and 33.
329=69\frac{3 \cdot2}{9} = \frac{6}{9}
However, we are over counting the (2,2)(2,2) configuration twice, and so, we subtract that one configuration from our total.
6919\frac{6}{9} - \frac{1}{9}.
Thus, our answer is (E) 59\boxed{\textbf{(E) }\frac{5}{9}}.
~ del-math.

Solution 5
This might take longer to solve, but you definitely will have the right answer.
You first list out all the possible combinations regardless if the product is even or odd.
11=1.1 \cdot 1 = 1.
12=2.1 \cdot 2 = 2.
13=3.1 \cdot 3 = 3.
21=2.2 \cdot 1 = 2.
22=4.2 \cdot 2 = 4.
23=6.2 \cdot 3 = 6.
31=3.3 \cdot 1 = 3.
32=6.3 \cdot 2 = 6.
33=9.3 \cdot 3 = 9.
There are 9 possible combinations, and 55 of the combinations’ products are even. So, our answer is (E) 59\boxed{\textbf{(E) }\frac{5}{9}}.
~MiracleMaths

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.