### July 17, 2009

The Standard Prelude makes tasks like this easy; we will use list-of (fold-of), sum, square, digits, ilog and permutations.

The first task is just a list comprehension that generates three-digit numbers and tests them for the required properties:

```> (list-of n     (n range 100 1000)     (zero? (modulo n 11))     (= (/ n 11)        (sum (map square (digits n))))) (550 803)```

The second task calls for us to dissect a number and stitch it back together, looping until we find the answer:

```> (let loop ((n 6))     (let ((m (+ (* 6 (expt 10 (ilog 10 n))) (quotient n 10))))       (if (= (* 4 n) m) n         (loop (+ n 10))))) 153846```

The third task is a bit harder. `In-position` and `two-consecutive` count the number of contestants in the specified positions; the list comprehension loops over the permutations of (A B C D E), testing each one.

```(define (in-position actual predicted)   (let loop ((a actual) (p predicted) (n 0))     (cond ((null? a) n)           ((equal? (car a) (car p))             (loop (cdr a) (cdr p) (+ n 1)))           (else (loop (cdr a) (cdr p) n)))))```

```(define (two-consecutive actual predicted)   (let loop ((a actual) (p predicted) (n 0))     (cond ((or (null? a) (null? p)) n)           ((null? (cdr p)) n)           ((null? (cdr a))             (loop actual (cdr p) n))           ((and (equal? (car a) (car p))                 (equal? (cadr a) (cadr p)))             (loop (cddr a) p (+ n 1)))           (else (loop (cdr a) p n)))))```

```> (list-of p     (p in (permutations '(a b c d e)))     (= (in-position p '(a b c d e)) 0)     (= (two-consecutive p '(a b c d e)) 0)     (= (in-position p '(d a e c b)) 2)     (= (two-consecutive p '(d a e c b)) 2)) ((e d a c b))```

You can run the suggested solution at http://programmingpraxis.codepad.org/JRGmt2wZ.

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### 5 Responses to “International Mathematical Olympiad”

1. iyo said

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

2. swaraj said

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

3. Vivek N said

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).

4. Skuba said

Problem 1 with Python:

```def main():
#Variables
num=0
digit1=0
digit2=0
digit3=0
sum_of_squares=0
upper_bound = 999
lower_bound = 100
count = lower_bound

#while loop runs through all three digit numbers
while count <= upper_bound:

#Only numbers divisible by 11 are evaluated
if count%11==0:

#convert number to string in order to seperate digits
num=str(count)
digit1 = num[0]
digit2 = num[1]
digit3 = num[2]

sum_of_squares = int(digit1)**2 + int(digit2)**2 + int(digit3)**2

if count//11 == sum_of_squares:
print (num)
count = count+1
else:
count = count+1
else:
count = count+1
main()
```

Results should be 550 and 803.

5. skuba713 said

Problem 2 with Python:

```def main():
#Variables
number=6
rearranged = 0
modified = 0
check = 0

#loop cycles through numbers until solution is found
while check == 0:

#put 6 in front and exclude last digit
rearranged = "6" + str(number)[0:len(str(number))-1]

modified = number * 4

#Check if solution is true
if int(rearranged) == int(modified):
check = 1
else:
#adding ten gives the next number ending in six
number = number + 10
print("The original number is:",number)
main()
```

Solution should be 153846