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Foreheadson
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Hi,

I didn't get the idea of splitting the numbers in 2 subsets:

Though my answer is incorrect but i would like to understand the flaw in my understanding :

Hundreds place could be written in 2c1 ways tens in 9C1 and unit place be filled in 5C1 ways

Is there any way of reaching the answer without splitting the entire numbers ?

walker
Let's xyz is our integer.

x e {8,9}
y e {0...9}
z e {1,3,5,7,9}

So, we have 1[x]*5[z]*(10-2)[y] = 40 integers that begin with 8 and 1[x]*4[z]*(10-2)[y] = 32 integers that begin with 9.
N = 40 + 32 = 72
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MuditKapoor
Hi,

I didn't get the idea of splitting the numbers in 2 subsets:

Though my answer is incorrect but i would like to understand the flaw in my understanding :

Hundreds place could be written in 2c1 ways tens in 9C1 and unit place be filled in 5C1 ways

Is there any way of reaching the answer without splitting the entire numbers ?
All the digits, in the three digit no. should be distinct

When you take 2C1, 9C1, and 5C1 for hundreds, tens, and units digits respectively, it includes cases such as 919, 877, 955, etc. where 2 digits are similar => hence this approach is incorrect.

Since you know that hundreds digit can either be 8, or 9, we can take each case separately.

8 _ 1 => 8 possible ways to fill tens digit
8 _ 3 => 8 possible ways to fill tens digit
8 _ 5 => 8 possible ways to fill tens digit
8 _ 7 => 8 possible ways to fill tens digit
8 _ 9 => 8 possible ways to fill tens digit

9 _ 1 => 8 possible ways to fill tens digit
9 _ 3 => 8 possible ways to fill tens digit
9 _ 5 => 8 possible ways to fill tens digit
9 _ 7 => 8 possible ways to fill tens digit
You cannot take 9 _ 9, since the digits have to be distinct

Thus, from above 8*9 = 72 possible odd integers between 800 and 1000, having each digit distinct.
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MuditKapoor
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Thank a ton Krunaal !! :)
Krunaal
MuditKapoor
Hi,

I didn't get the idea of splitting the numbers in 2 subsets:

Though my answer is incorrect but i would like to understand the flaw in my understanding :

Hundreds place could be written in 2c1 ways tens in 9C1 and unit place be filled in 5C1 ways

Is there any way of reaching the answer without splitting the entire numbers ?
All the digits, in the three digit no. should be distinct

When you take 2C1, 9C1, and 5C1 for hundreds, tens, and units digits respectively, it includes cases such as 919, 877, 955, etc. where 2 digits are similar => hence this approach is incorrect.

Since you know that hundreds digit can either be 8, or 9, we can take each case separately.

8 _ 1 => 8 possible ways to fill tens digit
8 _ 3 => 8 possible ways to fill tens digit
8 _ 5 => 8 possible ways to fill tens digit
8 _ 7 => 8 possible ways to fill tens digit
8 _ 9 => 8 possible ways to fill tens digit

9 _ 1 => 8 possible ways to fill tens digit
9 _ 3 => 8 possible ways to fill tens digit
9 _ 5 => 8 possible ways to fill tens digit
9 _ 7 => 8 possible ways to fill tens digit
You cannot take 9 _ 9, since the digits have to be distinct

Thus, from above 8*9 = 72 possible odd integers between 800 and 1000, having each digit distinct.
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I think we can do it this way;

2*9*4=72, hence C
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