## International Mathematical Olympiad

### July 17, 2009

These three problems from the International Mathematical Olympiad recently popped up on the web.

IMO 1960 Problem 01

Determine all three-digit numbers N having the property that N is divisible by 11, and N/11 is equal to the sum of the squares of the digits of N.

IMO 1962 Problem 01

Find the smallest natural number n which has the following properties:

(a) Its decimal representation has 6 as the last digit.

(b) If the last digit 6 is erased and placed in front of the remaining digits, the resulting number is four times as large as the original number n.

IMO 1963 Problem 06

Five students, A,B,C,D,E, took part in a contest. One prediction was that the contestants would finish in the order ABCDE. This prediction was very poor. In fact no contestant finished in the position predicted, and no two contestants predicted to finish consecutively actually did so. A second prediction had the contestants finishing in the order DAECB. This prediction was better. Exactly two of the contestants finished in the places predicted, and two disjoint pairs of students predicted to finish consecutively actually did so. Determine the order in which the contestants finished.

Your task is to solve the three problems. When you are finished, you are welcome to read or run a suggested solution, or to post your solution or discuss the exercise in the comments below.

Hi, thanks for problems. I think they are really simple to solve (you can use brute force to solve them without any difficulties :-)

The last one I even solved on paper. I share my solution:

When you took the second prediction and mark 2 consecutive contestants creating 2 disjoint pairs, you get 3 choices:

DA EC and B is somewhere else

DA CB and E is somewhere else

AE CB and D is somewhere else

Your goal is to place the “unpaired” contestant in good place.

1st choice:

B DA EC – wrong, none of them finished as it was predicted

DA B EC – B can’t be after A

DA EC B – all of them finished at it was predicted

2nd choice:

E DA CB – the correct answer!!! only C,B are at the right places

=========================================================

DA E CB – all of them finished at it was predicted

DA CB E – C is on the 3rd place – it can’t be because of the first prediction

3rd choice:

D AE CB – all of them finished at it was predicted

AE D CB – A is on the 1st place – it can’t be because of the first prediction

AE CB D – A is on the 1st place – it can’t be because of the first prediction

ruby solution for first problem (http://codepad.org/eq5LqB7o)

How do you solve these problems with a mathematical approach, for example if N is very large (something like 10^9, where bruteforce won’t work).

Problem 1 with Python:

Results should be 550 and 803.

Problem 2 with Python:

Solution should be 153846

I think both of those python approaches could be minified and simplified.

######### Problem 1

for a in range(1,10) :

for b in range(0,10) :

for c in range(0,10) :

N = a*100 + b*10 + c

if N % 11 is 0 and N/11 == a**2 + b**2 + c**2 :

print(N)

######### Problem 2

def n() :

exp = 2

while True :

start = 10**exp

for i in range(start+6,start*10,10) :

x = (i – 6)/10 + 6 * start

if 4 * i == x :

return i

exp += 1

print(n())

I’ll try problem 3 at another time

Something happened to the formatting……smh

Answers in Ruby 2.0