How many three digit numbers can be formed in which if one digit is 3 then it must be follwed by 7 and repetetion of digits is not allowed

• if 7 is to be followed by 3 , then the numbers will be either 37_ or _37.
  for 37_ ,we have 370,371,372,374,375,376,378,379.
  and for _37,we have 137,237,437,537,637,837,937.
  therefore i think the answer is 15
• 7 years agoHelpfull: Yes(12) No(8)
• The question says that if one number is 3 then it must be followed by 7, It does not says that we must include 3 in our formation of three digit number. Thus,
  Let's count the number of three digit numbers without having 3 in it -
  i.e. 8 x 8 x 7 = 448
  
  Now, let's count the number of three digit numbers having 3 in it and is followed by 7 -
  This would include two arrangements -
  1) 37_ (Total 8 arrangements can be done i.e. 370, 371, 372, 374, 375, 376, 378, 379).
  
  2) _37 (Total 7 arrangements can be done i.e. 137, 237, 437, 537, 637, 837, 937).
  
  Hence, total number of three digit numbers can be formed is 448+8+7 = 463.
• 7 years agoHelpfull: Yes(8) No(4)
• _37==>137,237,437,537,637,837,937 (7 numbers)
  37_==>370,371,372,374,375,376,378,379 (8 numbers)
  Total numbers that can be formed are 15
• 7 years agoHelpfull: Yes(5) No(2)
• Read the question carefully it says if there is a 3 then it is followed by 7. You have left cases where there is no 3
• 7 years agoHelpfull: Yes(4) No(3)
• Ans is 463. We have to consider other numbers also. I have seen all solutions and most of them are considering only number containg 3 and 7. We should consider other numbers also.
  Mr. Jayant Gope has given right solution.
• 1 year agoHelpfull: Yes(0) No(0)