2012 AIME II Problems/Problem 1
Find the number of ordered pairs of positive integer solutions to the equation .
Solving for gives us so in order for to be an integer, we must have The smallest possible value of is obviously and the greatest is so the total number of solutions is
Dividing by gives us . Solving for gives . The solutions are the numbers . There are solutions.
Because the x-intercept of the equation is , and the y-intercept is , the slope is . Now, notice the first obvious solution: (100,1). From it, we derive all the other solutions by applying the slope in reverse, i.e: Because the solutions are only positive, we can generate only 33 more solutions, so in total we have solutions.
Solution 4 (Designed to be very quick)
Note that a positive integer is divisible by 20 if it ends in 0.
Notice that if a multiple of 12 end in 2, 12 must be multiplied by an integer that ends in 1 or 6.
So let's start checking because 2012 ends in 2, same as 12.
When , .
When , . What is happening? Why doesn't, say, as values of , do work, while work for to be an integer (as seen in 2022 CEMC Cayley #21 (https://www.cemc.uwaterloo.ca/contests/past_contests/2022/2022CayleyContest.pdf, https://www.cemc.uwaterloo.ca/contests/past_contests/2022/2022CayleySolution.pdf)?)
Notice that we can break down into . So , where is a member of the set .
For a number to be divisible by 20, it must be divisible 10, and the quotient (that number divided by 10) must be divisible by 2. This means that the number must end in 0, and its tens digit must be even.
So notice that the tens digit cycle in: . Each of this is even (notice that when , , it cycles again).
So we know that all values of , which end in 1 or 6, that make works.
So values of that belong to work. Clearly, that is 34 integer values of . Therefore, the answer is .
You don't have to go through all this long thought process to make sure that values of ending in 1 or 6 work. You can just conjecture that they work, and then this will save a lot of time on tests. But look at it this way also: you got 3 hours. You probably only need to do 12 questions correct to go to USA(J)MO, considering that you got a 130+ on AMC 10/12. Time is not going to be as scarce as AMC. It depends on you how you look at it, but for me, this is a "cheap" and quick way to get to the answer (great as a test-taking strategy and solution!).
P.S.: I was on the honor roll for the 2022 CEMC Cayley contest above!
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