Fill in the gaps in the following sequence?
_, 4, _; _ 8, 9, 10, _ ; _, 14, 15, 16, _ ; _, _, _, _, _.
Answer: The series comprises sets of numbers flanked by prime numbers in ascending order – though some readers got there using a different rationale.
3, 4, 5 7, 8, 9, 10, 11 13, 14, 15, 16, 17 19, 20, 21, 22, 23
We do try to keep our questions as original and as Google-proof as possible. As a result, solutions are sometimes a bit less straightforward than intended – as was the case this week.
However, detecting our ‘prime sandwiches’ straight away was John Droogan, advanced polymers and composites, MegaChem (UK) Ltd, Caldicot, Monmouthshire, UK. His reply early Tuesday (day 1) was followed a little bit later by Franz Hochwimmer, sales & support inspection systems, Micro-Epsilon Messtechnik GmbH & Co. KG, Ortenburg, Germany; Paul Knutson, textile engineer, Timken Belts, Springfield, Missouri, USA; Thierry Montcalm, R&D and innovation manager, Soucy Techno, Canada; Jose Padron, material development specialist, Waterville TG Inc., Waterville, Québec, Canada; Ramasubramanian P, manager, marketing – mixer and LTKMPL products, Larsen & Toubro Ltd, Kanchipuram, Tamil Nadu, India; Yuichi (Joe) Sano, Sumitomo Electric Industries Ltd, Itami, Japan. Well done to all, and everyone else who had a go.