probability Like between 100 and 999 how many no have the prob that they
does not contain 7

• There are 900 numbers between 100 and 999.
  
  those that contain 7 are.....
  107, 117, 127, ........ -- 10 of these
  207, 217, 227, ........ -- 10 of these too
  307, 317, 327, .......
  ....
  9*10 numbers that end in 7.
  
  170, 171, 172, 173, 174, 175, 176, (NOT 177 WE ALREADY COUNTED IT), 178, 179 -- 9 of these
  270, 271, 272, 273, 274, 275, 276, (NOT 277 WE ALREADY COUNTED IT), 278, 279 -- 9 of these
  ...
  and so on
  
  so 9*9 numbers that have a middle digit of 7
  
  and finally
  700, 701, 702, 703, 704.................., but NOT
  (707, 717, 727,....) or (770, 771, 772, 773, ....)
  
  So that's (100 - 10 - 9) = 81 numbers that start with 7.
  
  We had 900 numbers.
  N = 900 - 90 - 81 - 81 = 648
  
  So the probability is 648/900 = 0.72
• 12 years agoHelpfull: Yes(1) No(0)