BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

counting and probability

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 1893
Joined: Sun May 30, 2010 11:48 pm
Thanked: 215 times
Followed by:7 members

by kvcpk » Tue Jun 15, 2010 2:16 am
If first digit is 9, there can be 100 numbers (10*10 )
If second digit is 9, there can be 100 numbers (10*9 ) [9 for first digit bcos it cant be zero]
If third digit is 9, there can be 100 numbers (10*9 ) [9 for first digit bcos it cant be zero]

so total 100+90+90 = 280 numbers

OA?
Top
User avatar
Legendary Member
Posts: 758
Joined: Sat Aug 29, 2009 9:32 pm
Location: Bangalore,India
Thanked: 67 times
Followed by:2 members

by sumanr84 » Tue Jun 15, 2010 5:37 am
duongthang wrote:find the number of 3-digit numbers abc that has exactly 1 digit 9 in it.
Notice the catch: exactly 1 digit 9..So on a similar note as kvcpk answer should be 225

9 X X => 9 * 9 = 81

X 9 X => 8 * 9 = 72

X X 9 => 8 * 9 = 72

Total = 225
I am on a break !!
Top
User avatar
Legendary Member
Posts: 1893
Joined: Sun May 30, 2010 11:48 pm
Thanked: 215 times
Followed by:7 members

by kvcpk » Tue Jun 15, 2010 7:40 am
sumanr84 wrote:
duongthang wrote:find the number of 3-digit numbers abc that has exactly 1 digit 9 in it.
Notice the catch: exactly 1 digit 9..So on a similar note as kvcpk answer should be 225

9 X X => 9 * 9 = 81

X 9 X => 8 * 9 = 72

X X 9 => 8 * 9 = 72

Total = 225
Thanks Suman for the correction. I missed it.
Top
Post new topic Post Reply
• Page 1 of 1