ERJ Brainteaser: July
13 Jul 2026
Each month, ERJ sets a weekly brainteaser, with questions of varying degrees of difficulty. Readers supplying the most accurate (and stylish) answers are then considered for the prestigious Brainiac of the Month title
Question 2: You cannot be series! (#11)
_?_, _?_ 125, 2,401, 161,051, 4,826,809, ..
Clues: Prime, Power…
Email your answer: correct replies on Friday.
Bonus question: Find the link
Anaya, Cardini, Grey, Montagu, Wellesley...
Clues: Nachos, Caesar...
Question 1: Not double
109.4, 218.7, 437.4, 874.9, _?_.
Clues: Track, Man in picture, Distance…
Answer: Just a few Brainiacs on the right track this week, and fewer still reached the correct finishing line - 1640.4 yards (Solutions below): Andrew Knox, Rubbond International, Ohé en Laak, The Netherlands; John Coleman, membership manager, Circol ELT, Dublin, Ireland; Dr Kate Burns, senior technologist and regulatory officer, Prisma Colour Ltd, Birch Vale, High Peak, Derbyshire, UK; John Bowen, BSc [Hons], CEng, MIMMM, consultant, Bromsgrove, UK. But we also accept the alternative interpretations from: Bharat B Sharma, technical director TWC Group (Techno Waxchem & Rajsha Chemicals), Kolkata, India; Hans-Bernd Luechtefeld, consultant, Germany; Kamila Staszewska, R&D / quality lead, Abcon Industrial Products Ltd, Cootehill, Co. Cavan, Ireland. Well done to all and everyone else who had a go.
SOLUTIONS
Andrew Knox
These numbers appear to be distances in yards of athletics track events in metres, listed in ascending order and starting with 100 metres. The next distance therefore will be 1640.4 yards (1500 metres), which is indeed the first distance which is not exactly twice the previous distance.
Why the next distance is not 1600 metres is a good question, as this would then have been very close to one mile (which is 1609.3 metres). (Of course, working in furlongs (1/8th mile, or 10 chains) would have been even simpler, one furlong being 201.2 metres, and half a furlong being 100.6 metres).
John Coleman
This is the classic athletics track distances (100 m, 200 m, 400 m, 800 m) converted from metres to yards (1 m ≈ 1.09361 yd).
Therefore, the next distance is the 1,500m (not double!) which equates to our answer - 1,640.4yds.
Dr Kate Burns
This sequence is the length of short and medium-distance running races in yards. The next distance in standard running events is 1500 m, or 1640.4 yards.
109.4 yards = 100 m
218.7 yards = 200 m
437.4 yards = 400 m
874.9 yards = 800 m
1640.4 yards = 1500m
ALTERNATIVELY
BB Sharma
A simple pattern is visible in the main values:
109.4 × 2 = 218.8 → given 218.7 (−0.1)
218.7 × 2 = 437.4 – Given 437.4 (0)
437.4 × 2 = 874.8 → given 874.9 (+0.1)
So, the next number could follow double with sequence of ((−0.1); 0, +0.1 which can be +0.2) - (sequence as -0.1; 0; +0.1; +0.2……)
So the next number would be 2x874.9+0.2 = 1749.8 +0.2 = 1750
Answer is 1750.
Kamila Staszewska
The pattern is race distances 100m, 200m, 400m, 800m expressed in yards.
The last distance is 1600m = 1749.8yd
Hans-Bernd Luechtefeld
1,749.8
These figures correspond exactly to the lengths of the Olympic track events, converted from meters to yards (1 meter ≈ 1.09361 yards): 100 meters = 109.4 yards, 200 meters = 218.7 yards, 400 meters = 437.4 yards, 800 meters = 874.9 yards, 1,600 meters (4 laps around the stadium) = 1,749.8 yards.
New teaser on Monday.
