How many four-digit numbers one can arrange using all the

You are close but the 2nd digit does not necessarily have to match the first one.

Is it 270 ?

9x1x10x1 = 90
9x10x1x1 = 90
9x1x1x10 = 90

Total = 270

Not quite. Much closer. Rethink your approach carefully.

It should be either 180 or 360? But which one?

After thinking a while, I think the answer is 180!
I need multiples of 75 minutes to finish the quant if I slove the
problems in such a speed

510.
My approach
Let's count all possible combinations, including those that begin with 0.
There are 6 ways to arrange a number, f.e. 3377, 3737, 3773, 7733, 7373, 7337.
And there are 10*1*9*1=90 ways to combine such number from digits.
Final step is to subtract numbers, beginning with 0
1*10*1*1*3=30
The ans is 6*90-30=510

You are double counting a few numbers here (for example, this method counts both 9 and 5, and 5 and 9, but they are the same because you already consider all combinations of 55 and 99.



Your final step is correct, but you are using wrong approach and calculation.

Right.

Then
3*90-30=240.

My Solution:

Method 1:
We have 2 different digits in each number. Let's call them A and B. Let's say we don't care about what order A and B are, so there are 10 * 9 / 2 = 45 difference pairings of A and B.

Now, let figure out how many way AA and BB can be put together. We have 4 digits, so there are 4! ways to arrange the four numbers, but the As and Bs are indistiguishable from each other so we need to adjust this by 2! twice. Hence, for a specific A and B the number of ways AABB can be combined is 4!/(2!2!) = (4*3)/(2*1) = 6.

So now we have 45 * 6 = 270 ways that AABB can be combined.

However, we cannot have 0 as a first digit. Since 0 is distributed the same as any of the other nine digits, exactly 10% of the numbers will start with zero and 90% will not. Hence the answer is 270 * .9 = 243.



Method 2:
There are 9 ways to pick the first number (excluding zero). Let's call this number A. For any given A, there are 9 ways to pick the other number B. So there are 81 ways to pick A as the first number and B as the other number. IF we set A to be the first digit, then there are 3 ways that the other 3 can be arranged: A-ABB, A-BAB, or A-BBA. Hence, there are 81 * 3 = 243 ways to make a four digit number with 2 paired numbers.

COMBINATORICS IS NOT ABOUT MEMORIZING FORMULAS, IT IS ABOUT COUNTING LOGICALLY AND SYSTEMATICALLY.

I invented this question occasionally, basing on the previous question in the forum. So I myself do not know the right answer.

My solution:

2 pairs can be arranged in 4!/(2!*2!)=6 ways
Now, count pairs: _ _ _ _

the first position -- 9 ways
any of the three left -- the same digit -- 1 way
the two left -- 9*1 ways

So, 9*1*9*1=81
I feel we have to multiply 6*81=486

But some doubling effect takes place, no?
Where is it?

You are double counting here. Say you get a 3 first, then a 5 second. It is still posible to get a 5 first and a 3 second, but they are both counted when you did the 4C2 calculation.

Even so, your method is still sloppy, since by using the 4C2 formula, you mix number that CAN be zero, with numbers that cannot be zero.

While you might back in to the correct answer, your solution in not good.

Sorry bro.

uch

I should have multiplied by 3 to get total numbers. Thanks anyway .