At first glance, the sequence looks almost meaningless: O = 10% / ONE H = 40% / ONE HUND = 70% / ?

It appeared on the BBC quiz show Only Connect, where contestants are expected to spot connections that can be remarkably difficult to see. This particular puzzle left both players and viewers scratching their heads, and it was later described as one of the show’s most elegant sequence questions.

If you can work out what belongs in the final space without looking at the answer, you have earned the right to feel rather pleased with yourself.

Can you spot the pattern?

The question was presented by Only Connect host Victoria Coren Mitchell during the show’s sequence round.

Contestants were given:

O = 10%
ONE H = 40%
ONE HUND = 70%
?

The challenge was to identify what should come next.

Unlike a conventional maths problem, there is no obvious calculation to perform. The trick is to recognise what the letters represent and why each group is paired with a particular percentage.

That makes the puzzle a classic Only Connect challenge. The programme is known for asking contestants to find links between clues that initially appear to have little in common, rewarding lateral thinking as much as straightforward knowledge.

And there is a reason this particular sequence feels so satisfying once the answer is revealed.

The answer is hiding in the words

The key is the phrase One Hundred.

The sequence gradually reveals more letters from those two words:

• O = 10%
• ONE H = 40%
• ONE HUND = 70%
• ONE HUNDRED = 100%

The percentage tells you how much of the phrase has been revealed.

So the missing entry is:

ONE HUNDRED = 100%

What initially looks like an arbitrary combination of letters and percentages is actually a simple progression towards the complete phrase.

The beauty of the puzzle is that the mathematics itself is minimal. The real challenge is recognising the relationship between the letters and the percentages.

A second maths puzzle proved just as tricky

If you enjoyed that one, there is another number puzzle that caused plenty of confusion online.

The sequence is:

2 + 3 = 10
8 + 4 = 96
7 + 2 = 63
6 + 5 = 66
9 + 5 = ?

At first, the equations appear to make no sense. Simply adding the two numbers clearly does not produce the answers shown.

People trying to solve the puzzle came up with several possibilities, including 14, 23 and 70.

But the pattern becomes clear once you look at the first number in each equation.

The trick behind the calculation

The answer is found by adding the two numbers first and then multiplying the result by the first number.

In other words:

X + Y = X × (X + Y)

That explains every example.

For 2 + 3:

2 × (2 + 3) = 2 × 5 = 10

For 8 + 4:

8 × (8 + 4) = 8 × 12 = 96

For 7 + 2:

7 × (7 + 2) = 7 × 9 = 63

And for 6 + 5:

6 × (6 + 5) = 6 × 11 = 66

So the final problem follows exactly the same rule:

9 × (9 + 5)

= 9 × 14

= 126

The answer is 126

The two puzzles demonstrate why these seemingly simple maths questions can be so frustrating. The arithmetic is relatively easy once the rule has been identified. The difficult part is discovering what the rule actually is.

And that is precisely the kind of lateral-thinking challenge that has made Only Connect famous for questions that can leave even confident quiz fans staring at the screen.