You are at a campus party where there are a total number of n people. The host asked everyone to put their phones in a bowl while walking in. A noise complaint ends the party abruptly, and everyone heads for the door, hastily grabbing their phones from the bowl Assume every guest has one and exactly one phone, and that they pick a phone at random (so that every assignment of a phone to a person is equally likely). What is the probability that: a. Every person gets their phone back? b. The first m persons to pick each get their own phones back? c. The first m persons to pick each get a phone belonging to the last m persons to pick? Hint: Try this thought experiment with a few choices of mand n to get a feel for the numbers that show up.)

1, , .

Step-by-step explanation:

probability =

Total number of persons in the party = n

a) Pr ( every person gets their phone back) = Pr (each person picks his phone ) multiplied by number of person

   = × n = 1.

     No of first m persons to pick = m

     No of last m persons to pick = 1 - m

b) Pr (first m persons to pick each gets their phones back) =

c) Pr( first m persons get a phone belonging to last m persons) =

starting with:    

5 + x - 12 = 2x - 7   

5 + x - 12 - x = 2x - 7 - x (subtract x from both sides)   

5 - 12 = x - 7 (combine and cancel x's from both sides)   

5 - 12 + 7 = x - 7 + 7 (add 7 to both sides)   

0 = x (add numbers together on both sides)       

so in order for the equation to be true, x must be 0.   

trying x = -0.5   

5 + x - 12 = 2x - 7 (original equation)   

5 + -0.5 - 12 = 2(-0.5) - 7 (plug in value of x)   

5 + -0.5 - 12 = -1 - 7 (did multiplication)   

-7.5 = -8 (did addition on both sides)       

since -7.5 is obviously not equal to -8, the value of -0.5 for x is wrong.   

trying x = 0   

5 + x - 12 = 2x - 7 (original equation)   

5 + 0 - 12 = 2(0) - 7 (plug in value of x)   

5 + 0 - 12 = 0 - 7 (did multiplication)   

-7 = -7 (did addition on both sides)       

since -7 equals -7, that demonstrates that the value of 0 for x is correct.   

trying x = 1   

5 + x - 12 = 2x - 7 (original equation)   

5 + 1 - 12 = 2(1) - 7 (plug in value of x)   

5 + 1 - 12 = 2 - 7 (did multiplication)   

-6 = -5 (did addition on both sides)       

since -6 is obviously not equal to -5, the value of 1 for x is wrong.