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 Math Help −> Puzzles −> AD 2004 state championship logic quiz −> Question 20 −> Answer 

Academic Decathlon 2004 State Championship Logic Quiz

Question 20: EIGHT + THREE + NINE = TWENTY

What digit does each letter represent so that the following addition is correct?

  E I G H T
+ T H R E E
+   N I N E
T W E N T Y

Hint: T=1, R=2G

Answer:

  8 5 2 9 1
+ 1 9 4 8 8
+   3 5 3 8
1 0 8 3 1 7

As the hint tells you, T=1.  You should be able to figure that out pretty quickly anyway, because it was carried into the hundred-thousands place.  No matter how big E is, you can't carry anything more than 1 into the leftmost place.

In this solution, I'll use the shorthand that everything is modulo 10 unless otherwise specified.  That is, if I say that T+E+E=9, you'll know I mean just that the one's digit is 9.

Also, I'll write the carry in parentheses.  For example, the tens column has H, E, and N adding up to T.  The carry into the tens column was 1, so I'll write this as (1)+H+E+N=T.

Now that the ground rules are out of the way, let's get to the puzzle...

E has to be a pretty big digit, at least 7, because when you add 1 to it (plus whatever got carried into the ten-thousands place) you get a 2-digit number.  (But E can't be 9, because then T+E+E=Y, and so both E and Y would be 9)

W, then, has to be 0, because (x)+E+T is at most (2)+8+1=11.  But it can't be 11, because then W would be 1, and the 1 is already taken.  So (x)+E+1 is at most 10, so W is 0.

Y is 5 or 7, depending on whether E is 7 or 8, based on the one's place.  That is, if E=7 then T+E+E =1+7+7=5.  If E=8 then T+E+E=1+8+8=7.

Here's what we know so far...  If E=7 then Y=5

Case 1:

  7 I G H 1
+ 1 H R 7 7
+   N I N 7
1 0 7 N 1 5

But this case doesn't work because even if 2 is carried into the thousands column (2)+I+H+N can't be as high as 27 because I, H, and N can't be any more than 9, 8, and 6, in some order.  (2)+9+8+6=25, not enough to both leave a 7 in the thousands place of the sum and generate the necessary carry of 2 into the ten-thousands place.  So Case 1 is a non-starter.

If E=8 then Y=7:

Case 2:

  8 I G H 1
+ 1 H R 8 8
+   N I N 8
1 0 8 N 1 7

In Case 2, 1 was carried into the tens place, so (1)+H+8+N=11 or 21.  H and N can't be 1 or 0, so 11 is too low.  (1)+H+8+N=21, so H+N=12.  4+8 is ruled out because 8 is already taken (E=8).  5+7 is ruled out because Y=7.  So H+N is 3+9, in some order.  Here are the two orders:

Case 2a:

  8 I G 3 1
+ 1 3 R 8 8
+   9 I 9 8
1 0 8 9 1 7

All the digits are taken except for 2, 4, 5, and 6.  From the hundreds column we see that (2)+G+R+I=9, so G+R+I=7 (or 17 or 27).  The smallest possibility for G+R+I is 2+4+5=11, and the largest possibility is 4+5+6=15, so G+R+I=7 is impossible.  Case 2a is ruled out.

Case 2b:

  8 I G 9 1
+ 1 9 R 8 8
+   3 I 3 8
1 0 8 3 1 7

In case 2b, 2, 4, 5, 6 are left for G, R, and I, and (2)+G+R+I=13, so they are 2, 4, and 5.  In the ten-thousands place, (1)+I+9+3=18, so I=5.

Case 2b:

  8 5 G 9 1
+ 1 9 R 8 8
+   3 5 3 8
1 0 8 3 1 7

The only digits left for R and G are 2 and 4.  This is where the second hint narrows the choice down to one possibility.  Since R=2G, we know R=4 and G=2, giving us the final answer.

 

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