

Line 1: 
Line 1: 
−  == Problem ==
 +  #REDIRECT [[2010_AMC_12A_Problems/Problem_16]] 
−  Bernardo randomly picks 3 distinct numbers from the set <math>\{1,2,3,4,5,6,7,8,9\}</math> and arranges them in descending order to form a 3digit number. Silvia randomly picks 3 distinct numbers from the set <math>\{1,2,3,4,5,6,7,8\}</math> and also arranges them in descending order to form a 3digit number. What is the probability that Bernardo's number is larger than Silvia's number?
 
−   
−  <math>\textbf{(A)}\ \frac{47}{72} \qquad \textbf{(B)}\ \frac{37}{56} \qquad \textbf{(C)}\ \frac{2}{3} \qquad \textbf{(D)}\ \frac{49}{72} \qquad \textbf{(E)}\ \frac{39}{56}</math>
 
−   
−  == Solution ==
 
−  We can solve this by breaking the problem down into <math>2</math> cases and adding up the probabilities.
 
−   
−   
−  Case <math>1</math>: Bernardo picks <math>9</math>.
 
−  If Bernardo picks a <math>9</math> then it is guaranteed that his number will be larger than Silvia's. The probability that he will pick a <math>9</math> is <math>\dfrac{3}{9} = \frac{1}{3}</math>.
 
−   
−   
−  Case <math>2</math>: Bernardo does not pick <math>9</math>.
 
−  Since the chance of Bernardo picking <math>9</math> is <math>\frac{1}{3}</math>, the probability of not picking <math>9</math> is <math>\frac{2}{3}</math>.
 
−   
−  If Bernardo does not pick 9, then he can pick any number from <math>1</math> to <math>8</math>. Since Bernardo is picking from the same set of numbers as Silvia, the probability that Bernardo's number is larger is equal to the probability that Silvia's number is larger.
 
−   
−  Ignoring the <math>9</math> for now, the probability that they will pick the same number is the number of ways to pick Bernardo's 3 numbers divided by the number of ways to pick any 3 numbers.
 
−   
−  We get this probability to be <math>\frac{3!}{8\cdot{7}\cdot{6}} = \frac{1}{56}</math>
 
−   
−  (This was confusing for me, so think of it like this: Bernardo chooses his number. Silva has one out of the total number of options to get the same number. The total number of options is 8 choose 3: <math>\frac{8\cdot{7}\cdot{6}}{3!} = 56</math> . There are 8 numbers and you choose three to make a threedigit number. Setting it up to be descending doesn't affect the number of ways there are.
 
−  )
 
−   
−  Probability of Bernardo's number being greater is
 
−  <cmath>\frac{1\frac{1}{56}}{2} = \frac{55}{112}</cmath>
 
−   
−  Factoring in the fact that Bernardo could've picked a <math>9</math> but didn't:
 
−   
−  <cmath>\frac{2}{3}\cdot{\frac{55}{112}} = \frac{55}{168}</cmath>
 
−   
−  Adding up the two cases we get <math>\frac{1}{3}+\frac{55}{168} = \boxed{\frac{37}{56}\ \textbf{(B)}}</math>
 
−   
−  == See also ==
 
−  {{AMC10 boxyear=2010numb=17numa=19ab=A}}
 
−   
−  [[Category:Introductory Combinatorics Problems]]  
−  {{MAA Notice}}
 