Reasoning
In 36 hours, Alex could empty 2 lots of bookshelves, Billie could also empty 2 lots of bookshelves, and Drew could refill 1 lot of bookshelves.
So, 2 + 2 − 1 = 3 lots of bookshelves in 36 hours. Which is 12 hours for one lot of bookshelves.
Alternative Reasoning
Alex and Billie can each empty 1/18 of the bookshelves in 1 hour, but Drew will refill 1/36 of the bookshelves in one hour.
So in one hour we have 1/18 + 1/18 − 1/36 = 1/12.
So it would take 12 hours to empty the bookshelves.
A million grains of sand is a heap. If we remove one grain of sand from this heap, we will still have a heap.
We can now keep repeating (2) until we only have a single grain of sand remaining.
Is this a heap? Clearly not. But what went wrong with our thinking?
This is called the Sorites paradox (soros being Greek for "heap") and is a classic paradox that has no real answer.
Both (1) and (2) are true, and we can indeed keep removing one grain of sand until we have a single grain remaining. If we remove one more grain, we're left with nothing, is this still a heap?
From Laurel and Hardy, the names have been merged.
?
Puzzle 204
My BrainBashers electronic world atlas has developed a fault, I did a listing of miles from Scotland to particular countries, and here is the result:
England 12,000 miles
Peru 6,000 miles
Egypt 9,000 miles
Iceland 11,000 miles
