Countdown Game Show

Then, when I was 15, a new station was launched with imaginative name of Channel 4. The first program to air on this new channel was a game show called Countdown. Countdown is still shown today and is one of the longest running game shows in the World. At the time of writing this article, over 5,000 episodes have been broadcast.

The show was not entirely original in concept, being based on a slightly earlier game shown in France called Des chiffres et des lettres (Numbers and Letters).

The Countdown game tests contestants' numeracy and word-making skills. In the ‘letters’ rounds of the game, players have to create the longest word possible from a random selection of letters (a little bit like Scrabble) picked from vowels and consonants.

In the ‘numbers’ round of the game, players have to combine six selected numbers (using just the four basic arithmetic operators) to get as close as possible to a randomly generated total. It’s is the numbers game that I’m going to talk about in this post.

Here's an example of the game in action. In this instance, one number was selected from the large set, and the rest from the small set.

{ 50 , 8 , 3 , 7 , 2 , 10 }

The randomly selected target was 556.

On the show, contestants' declared solutions are confirmed by one of the hosts (who then goes on to reveal a better/alternate solution if one of the contestants does not get an exact solution). For 26 years, this host role was performed by the talented Carol Vorderman.

Being a numbers geek, I wanted to know many things:

• Are all problems solvable for a perfect score? Does there exist a three digit number that is not achievable by any combination of digits? If not, what is the hardest number to achieve?

• Is there a perfect set of numbers that allows the solving of any target 100-999?

• What is the best strategy? Is it better to take 0,1,2,3 or 4 large numbers?

• What is the ‘simplest’ solution to every problem (If there are multiple ways to solve any particular problem, what is the solution that requires the smallest number of donor numbers)

• What is the distribution of the solutions?

• Are there some interesting pieces of trivia found from analysing the problem?

A trap that needs to be avoided is the ordering the operators should be applied. For example 10-(7-2) gives a very different solution to (10-7)-2, just as (10×4)-2 is very different from 10×(4-2). In RPN notation these distinctions are obvious (See table).

• There is no benefit in multiply by 1 at any stage. This just makes a longer solution.

• Similarly, there is no benefit of dividing by 1 at any stage.

• If at any time the intermediate solution becomes negative, we can instantly bail on that solution.

• Similarly if the intermediate solution becomes non-integer.

• Slightly more subtly, if there is an intermediate calculation where a÷b=b then this is redundant because this is just a longer form of using b.

• Likewise for a-b=b.

• You need to make sure you catch any potential divide by zero calculation that might happen as you test for possible solutions.

• It's very tempting to think about using 16 bit Integers to store intermediate but this is wrong for a couple of reasons. The highest possible intermediate accumulator total encountered is 100×75×50×25×10×10 = 937,500,000. This would cause an overflow.

• Notwithstanding the above, the highest intermediate total seen in a valid solution is 99,600 which happens when trying to obtain a total of 996 from the set {3,3,25,50,75,100}.
  
  The solution for this, and it is a unique solution for this total, is ((((50+3)×25)+3)×75)÷100=996 If this came up on the show, and you'd managed to work this out, I'm sure you'd be proud!
  
  The most convoluted I've heard about in the history of the show is this one for 952, followed closely by 821 (from an Australian version of the show). Pretty impressive!

• The 13,243 possible combinations that the tiles can be selected can be combined to make 10,871,986 distinct solutions of tiles and target numbers.

• There is no target number that it is impossible to make, given the correct numbers.

• Only one set of number {1,1,2,2,3,3} cannot make any target solution (as it’s not possible to make any three digit number from this set of numbers).
  
  If you were unlucky enough to draw these numbers there is nothing you could do. The hight possible total achievable from these six numbers is 81 = (2+1)×(2+1)×3×3. Even the lowest target of 100 is more than 10 away from this, so no score is possible.

• The ‘hardest’ total to make is 947. Just 9,017 of the 13,243 combinations can make it (68.09%).

• There are four target numbers that are the ‘easiest’ total to make. These are: 100, 102, 104 and 108 both of these can be made by 13,240 of the possible 13,243 tile combinations (99.98%). Of the three solutions missing, one should not be a surprise, we’ve already discussed it, and this is {1,1,2,2,3,3}. Since it’s not possible to make any target number from this set, it has to be missing by definition! Related to this is the set {1,1,2,2,3,4} which can only can one possible total, which is 108 (1+2)×((1+2)×(3×4)). Finally numbers in the set {1,7,7,8,8,9} cannot be combined to make 100.
  
  For 102 and 104, the impossible sets are {1,1,2,2,3,3} {1,1,2,2,3,4} {1,1,2,2,3,5}
  
  For 108, the impossible sets are {1,1,2,2,3,3} {1,1,5,5,50,75} {1,1,10,10,50,75}

• Out of the 13,243 possible combinations, a surprising 1,226 of them can solve any problem from 100-999 inclusive (9.26%). Below are the first few resutls of a set that can make all 900 possible target numbers. It is the set {2,7,9,10,25,50}. The first few solutions are shown below (click link after for complete list):

Of the 1,226 perfect solution sets (sets of numbers that can solve any problem), only five of them contain no large number (for the shortest solution). There is no perfect number set that contains all four large numbers. If you select just one large number, you marginally increase your chances of getting a set of numbers that can solve any problem (cf. selecting two large numbers).

The table on the left shows the distribution of large number counts in all the perfect solutions (for the shortest possible solution).

Try your hand at solving Countdown numbers problems. If you want to find out solutions, I've put a solver online.