Problem 22 · 2017 Math Kangaroo
Stretch
Counting & Probability
careful-countingcomplementary-counting
A small zoo has a giraffe, an elephant, a lion and a turtle. Susi wants to visit exactly two of the animals today but does not want to start with the lion. How many different possibilities does she have, to visit the two animals one after the other?
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Answer: D — 9
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Hint 1 of 2
Count ordered visits of two different animals, then remove the forbidden starts.
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Hint 2 of 2
There are 4 x 3 ordered pairs; throw out the ones beginning with the lion.
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Approach: count ordered pairs, then subtract those starting with the lion
- Choosing two animals in order gives 4 x 3 = 12 possibilities.
- Of these, the ones starting with the lion number 1 x 3 = 3.
- Allowed possibilities: 12 - 3 = 9.
- So she has 9 possibilities.
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