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Title: Probability
Description: Breaks down the little elements to understand the application of probability; furthermore, twenty-four questions included with solutions!

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     GMATPREPNOW  

 
Probability    
 
1
...
 What  is  the  
probability  that  the  selected  number  is  either  even  or  prime?    
 
P(A)  =  (#  of  even  or  prime  outcome)  /  (  total  number  of  possible  outcomes)    
     4/7    
 
 
2
...
 What  is  the  probability  that  
the  sum  of  the  two  numbers  is  11?    
 
                             P(Sum  is  11)  =  (Number  of  Pairs  with  sum  11)/  (total  number  of  pairs)    
 
 
4/40  =  1/10    
 
 
3
...
1    
 
 
 
=  0
...
Two-­‐monitor   quality   of   an   egg   packaging   plant,   workers   randomly   select  
eggs   for   testing
...
04;   the  
probability   of   selecting   an   extra   large   egg   0
...
07
...
On   any   given   toss,   the   probability   is   0
...
 If  the  coin  is  tossed  5  times  what  is  the  probability  that  exactly  2  of  the  
tosses  turn  up  heads?  
P(2  Heads)  =  P(HHTTT  or  HTHTT  or  THTHT  or  …)    
 
P(HHTTT)  =  9/10  x  9/10  x  1/10  x  1/10  x  1/10    
 
81/  100,000  +  81/  100,000  +  81/  100,000  +  …
...
A  drawer  contains  six  socks
...
 Probability  is  less  than  0
...
   
B
...
4  that  both  socks  will  be  white
...
3    =  B/6  <  0
...
33  (WRONG)  
B  =  1  à  P(Black)  =  1/6  =  0
...
4  
W/6  x  W-­‐1/5  >  0
...
4  (Wrong)    
 
 
 
7
...
 If  a  point  is  randomly  selected  from  the  region  inside  
the   larger   circle,   what   is   the   probability   that   the   point   will   be   inside   the  
smaller  circle?  
Area  of  Small  Circle  /  Area  of  Bigger  Circle    
π(2)  ^2  /  π(10)^2  =  4π/100π  =  4/100  =  1/25  
 
 

8
...
 What  is  the  probability  that  a  ball  
selected  at  random  will  be  green  ball
...
   There  are  15  more  red  balls  than  green  balls
...
 There  are  twice  as  many  red  balls  as  there  are  green  balls
...
Box  contains  12  green  balls,  12  red  balls,  and  1  yellow  ball
...
Box   contains   N   balls,   3   of   which   are   red
...
 
What  is  the  value  of  N  ?  
 
P(both  red)  =  1/12  
P(Red   1   AND   Red   2)   =   1/12   à   Dependent,   therefore   P(A   AND   B)   =   P(A)   x  
P(B|A)  
 3/N  x  2/N-­‐1  =  1/12    
N^2  –  N  =  72    
(N-­‐9)  (N+8)  =  0  =>  N=  9,  -­‐8    
 
 
 
 

 
Counting    
 
 
1
...
In  how  many  ways  can  6  identical  coins  be  distributed  among  Alex,  Bea,  
and  Chad?  Note:  Some  people  may  receive  zero  coins  
 
1st  –  {6,0,0}  
 
(6  –  1)(5-­‐2)(4-­‐3)(3-­‐4)(2-­‐5)(1-­‐6)(0-­‐7)  
nd  –  {5,1,0}  
2
         {5,0,1}  
So  on…
...
How   many   different   cars   are   possible?   If   Power   (   Gas,   Electric,   Hybrid)  
then   color   (Red,   Black,   Blue)   and   the   transmissions   are   (Automatic,  
Standard)    

 TOTAL   18   Possibilities   =   3   (power)   x   3  
(Color)x  2  (Trans)    
 
4
...
  In   how   many   different   ways   can   all   five   children   be  
seated  in  a  row  if  the  girls  must  sit  on  the  end  chairs?  

 
 
 
5
...
 How  many  positive  3  digits  odd  number  
can  be  created  if  each  number  cannot  contain  repeated  digit?  
 
Fundamental  Counting  Principle  =  5  x  4  x  3  =  40  total    
 
6
...
 In  how  many  ways  can  the  test  be  completed  if  every  
question  is  answered?    
 
5  x  5  x  5  x  5  =  625    
 
7
...
  If   each   person  
finishes   the   race,   and   there   are   no   ties
...
In   how   many   ways   can   we   arrange   the   9   letters   in   the   word  
WONDERFUL?    
 
9!  =  362,880  ways  
 
9
...
Chole   has   3   different   math   books,   2   different   art   books   and   2   different  
history  books
...
In  how  many  ways  can  the  five  letters  J,  K,  L,  M,  and  N  be  arranged  such  
that  L  is  not  in  the  middle?  
#   of   ways   to   follow   restriction   =   #   of   ways   to   ignore   restriction   -­‐   #   of  
ways  to  break  restriction    
Ignore  =  5  x  4  x  3  x  2  x  1  =  120    
Break  =  4  x  3  x  1  x  2  x  1  =    24    
120  –  24  =  96    
 

12
...
In  how  many  different  ways  can  we  arrange  the  5  letters  in  NANNY?  
N!/  (A!)  (B!)  (C!)    
 
3N’s  1  A  1  Y  =  5  TOTAL  =  n  
5!/  3!  X  1!  X  1!  =  20    
 
14
...
From   your   group   of   5   friends   (A,   B,   C,   D,   and   E)   you   must   select   3   friends  
to   join   you   on   a   campaigning   trip
...
There  are  50  students  in  a  certain  school
...
A   certain   pizza   restaurant   offers   12   toppings
...
How  many  3-­‐digit  numbers  greater  than  399  end  with  5  or  7?  
 
 
1st  stage  (x>  3)  6  x    10  x  2  =  120  numbers  

19
...
 If  everyone  in  the  room  shakes  hands  with  
everyone  else  in  the  room,  how  many  handshakes  takes  place?  
 
8C2  =  8!  /  2!  (8-­‐2)!  =  28    
 

 
 
 

 
20
...
  How   many   different   3  
toppings  pizzas  are  possible,  if  the  order  of  the  3  selected  toppings  does  
not  matter?  
 8C3  =  8  x  7  x  6  /  3  x  2  x  1  =  8  x  7  =  56  

21
...
   
11C2  =  11!/  2!  X  (11-­‐2)!  =  55  
22
...
   
7C3  =  7!/3!  X  (7-­‐3)!  =  35  
23
...
  In   how   many   different   ways   can   the   teacher   select  
a  committee  consisting  of  4  students?  
 
8C4  =  8  x  7  x  6  x  5/  4!  =  70    
 
 
24
...
  There   are   2   red   buttons,   2   blue   buttons  
and   2   green   buttons
...
A  couple  is  planning  an  evening  out
...
  How   many   different   ways   can   they   plan   their   evening   if   they  
choose  one  of  each?  
 
 
4  x  5  x  3  =  60    
 
 


Title: Probability
Description: Breaks down the little elements to understand the application of probability; furthermore, twenty-four questions included with solutions!