2006 AIME II Problems/Problem 7
There are numbers up to 1000 that have 0 as their units digit. All of the other excluded possibilities are when or have a 0 in the tens digit, and since the equation is symmetric, we will just count when has a 0 in the tens digit and multiply by 2 (notice that the only time both and can have a 0 in the tens digit is when they are divisible by 100, which falls into the above category, so we do not have to worry about overcounting).
Excluding the numbers divisible by 100, which were counted already, there are numbers in every hundred numbers that have a tens digit of 0 (this is true from 100 to 900), totaling such numbers; considering also and we have . Therefore, there are such ordered pairs.
Let and be 3 digit numbers:
cde +fgh ---- 1000
and must add up to , and must add up to , and and must add up to . Since none of the digits can be 0, there are possibilites if both numbers are three digits.
There are two other scenarios. and can be a three digit number and a two digit number, or a three digit number and a one digit number. For the first scenario, there are possibilities (the two accounting for whether or has three digits) and for the second case there are possibilities. Thus, thus total possibilities for is .
We first must notice that we can find all the possible values of between and and then double that result.
When there are possible solution for so that neither nor has a zero in it, counting through , through , ..., through . When there are possible solution for a so that neither a nor b has a zero in it, counting through , through , ..., through . This can clearly be extended to where is an integer and . Thus for there are = possible values of .
Thus when there are possible values of and .
Doubling this yields .
Solution 4 (Similar to Solution 2)
We proceed by casework on the number of digits of
Case 1: Both and have three digits
We now use constructive counting. For the hundreds digit of we see that there are options - the numbers through (If that means that will be a two digit number, and if will have two digits). Similarly, the tens digit can be as well because a tens digit of is obviously prohibited and a tens digit of will lead to a tens digit of in the other number. The units digit can be anything from Hence, there are possible values in this case.
Case 2: (or ) has two digits
If has two digits, the only restrictions are that the units digit must not be and the tens digit must not be (because then that would lead to beginning with ). There thus are possibilities for and we have to multiply by because there are the same number of possibilities for Thus, there are possible values in this case.
Case 3: (or ) has one digit
This is easy -- can be anything from to for a total of possible values. We multiply this by to account for the single digit values, so we have possible values for this case.
Adding them all up, we get and we're done.
Solution by Ilikeapos
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