38. If 6 different numbers are to be selected from integers 0 to 6, how many 6-digit even integers greater than 300,000 can be composed?
OA is 1680 ?????
Q-38
This topic has expert replies
-
- Senior | Next Rank: 100 Posts
- Posts: 65
- Joined: Thu Jul 02, 2009 10:38 pm
- Location: Bangalore
- Thanked: 1 times
-
- Master | Next Rank: 500 Posts
- Posts: 399
- Joined: Wed Apr 15, 2009 3:48 am
- Location: india
- Thanked: 39 times
-
- Senior | Next Rank: 100 Posts
- Posts: 43
- Joined: Tue Mar 31, 2009 9:54 am
- Thanked: 2 times
-
- Master | Next Rank: 500 Posts
- Posts: 191
- Joined: Sat Jun 26, 2010 9:27 am
- Thanked: 6 times
- Followed by:2 members
I calculated as follows:
7 usable digits (0,1,2,3,4,5,6)
We need to find numbers greater than 3,00,000. hence,
1) [3] [] [] [] [] [last digit can be any of 0,2,4,6.ie. 4 ways to select last digit]
2) [4] [] [] [] [] [any of 4 even digits]
3) [5] [] [] [] [] [any of 4 even digits]
4) [6] [] [] [] [] [any of 4 even digits]
We have used up 2 out of 7 digit, we are left with 5 digits
Total number of ways to select 1st selection = 1*5*4*3*2*4 = 480 ways
but this is similar to remaining 3 cases...hence each case will have 480 ways
total number of ways = 480*4 = 1920 ways.
Is this correct? if not what am I missing and where?
7 usable digits (0,1,2,3,4,5,6)
We need to find numbers greater than 3,00,000. hence,
1) [3] [] [] [] [] [last digit can be any of 0,2,4,6.ie. 4 ways to select last digit]
2) [4] [] [] [] [] [any of 4 even digits]
3) [5] [] [] [] [] [any of 4 even digits]
4) [6] [] [] [] [] [any of 4 even digits]
We have used up 2 out of 7 digit, we are left with 5 digits
Total number of ways to select 1st selection = 1*5*4*3*2*4 = 480 ways
but this is similar to remaining 3 cases...hence each case will have 480 ways
total number of ways = 480*4 = 1920 ways.
Is this correct? if not what am I missing and where?
-
- Senior | Next Rank: 100 Posts
- Posts: 98
- Joined: Sat Feb 06, 2010 11:56 pm
- Thanked: 5 times
Almost correct...
But,
We need to find numbers greater than 3,00,000. hence,
1) [3] [] [] [] [] [last digit can be any of 0,2,4,6.ie. 4 ways to select last digit]
2) [4] [] [] [] [] [any of 3 even digits] (already chosen one even digit)
3) [5] [] [] [] [] [any of 4 even digits]
4) [6] [] [] [] [] [any of 3 even digits]
Hence
for 1) we have 480 combinations- 5.4.3.2.4
for 2) we have 360 combinations- 5.4.3.2.3
..
..
so in all 480*2+360*2=1680
But,
We need to find numbers greater than 3,00,000. hence,
1) [3] [] [] [] [] [last digit can be any of 0,2,4,6.ie. 4 ways to select last digit]
2) [4] [] [] [] [] [any of 3 even digits] (already chosen one even digit)
3) [5] [] [] [] [] [any of 4 even digits]
4) [6] [] [] [] [] [any of 3 even digits]
Hence
for 1) we have 480 combinations- 5.4.3.2.4
for 2) we have 360 combinations- 5.4.3.2.3
..
..
so in all 480*2+360*2=1680