Problems with reversed digits

The CAT asks a fair number of questions about the properties of two-digit numbers whose tens and units digits have been reversed. Because these questions pop up so frequently, it’s worth spending a little time to gain a deeper understanding of the properties of such pairs of numbers. Like much of the content on the CAT, we can gain understanding of these problems by simply selecting random examples of such numbers and analyzing and dissecting them algebraically. Let’s do both. First, we’ll list out some random pairs of two-digit numbers whose tens and units digits have been reversed: {34, 43}; {17, 71}; {18, 81.} Now we’ll see if we can recognize a pattern when we add or subtract these figures. First, let’s try addition: 34 + 43 = 77; 17 + 71 = 88; 18 + 81 = 99. Interesting. Each of these sums turns out to be a multiple of 11. This will be true for the sum of any two two-digit numbers whose tens and units digits are reversed. Next, we’ll try subtraction: 43 – 34 = 9; 71 – 17 = 54; 81 – 18 = 63; Again, there’s a pattern. The difference of each pair turns out to be a multiple of 9. Algebraically, this is easy enough to demonstrate. Say we have a two-digit number with a tens digit of “a” and a units digit of “b”. The number can be depicted as 10a + b. (If that isn’t clear, use a concrete number to illustrate it to yourself. Let’s reuse “34”. In this case a = 3 and b = 4. 10a + b = 10*3 + 4 = 34. This makes sense. The number in the “tens” place should be multiplied by 10.) If the original number is 10a + b, then swapping the tens and units digits would give us 10b + a. The sum of the two terms would be (10a + b) + (10b + a) = 11a + 11b = 11(a + b.) Because “a” and “b” are integers, this sum must be a multiple of 11. The difference of the two terms would be  (10a + b) – (10b + a) = 9a – 9b = 9(a – b) and this number will be a multiple of 9. Now watch how easy certain official GMAT questions become once we’ve internalized these properties: The positive two-digit integers x and y have the same digits, but in reverse order. Which of the following must be a factor of x + y? A) 6 B) 9 C) 10 D) 11 E) 14 If you followed the above discussion, you barely need to be conscious to answer this question correctly. We just proved that the sum of two-digit numbers whose units and tens digits have been reversed is 11! No need to do anything here. The answer is D. Pretty nice. Let’s try another, slightly tougher one: If a two-digit positive integer has its digits reversed, the resulting integer differs from the original by 27. By how much do the two digits differ? A) 3 B) 4 C) 5 D) 6 E) 7 This one is a little more indicative of what we’re likely to encounter on the actual GMAT. It’s testing us on a concept we’re expected to know, but doing so in a way that precludes us from simply relying on rote memorization. So let’s try a couple of approaches. First, we’ll try picking some numbers. Let’s use the answer choices to steer us. Say we try B – we’ll want two digits that differ by 4. So let’s use the numbers 84 and 48. Okay, we can see that the difference is 84 – 48 = 36. That difference is too big, it should be 27. So we know that the digits are closer together. This means that the answer must be less than 4. We’re done. The answer is A. (And if you were feeling paranoid that it couldn’t possibly be that simple, you could test two numbers whose digits were 3 apart, say, 14 and 41. 41-14 = 27. Proof!) Alternatively, we can do this one algebraically. We know that if the original two-digit numbers were 10a +b, that the new number, whose digits are reversed, would be 10b + a. If the difference between the two numbers were 27, we’d derive the following equation: (10a + b) – (10b + a) = 27. Simplifying, we get 9a – 9b = 27. Thus, 9(a – b) = 27, and a – b = 3. Also not so bad. Takeaway: Once you’ve completed a few hundred practice questions, you’ll begin to notice that a few strategies are applicable to a huge swath of different question types. You’re constantly picking numbers, testing answer choices, doing simple algebra, or applying a basic number property that you’ve internalized. In this case, the relevant number property to remember is that the sum of two two-digit numbers whose units and tens digits have been reversed is always a multiple of 11, and the difference of such numbers is always a multiple of 9. Generally speaking, if you encounter a particular question type more than once in the Official Guide, it’s always worth spending a little more time familiarizing yourself with it.