Last week, I travelled from Birmingham to Glasgow, the long way round (on a bicycle!).
On day one, I travelled one quarter of the distance.
On day two, I travelled one half of the remaining distance.
On day three, I travelled three quarters of the remaining distance.
Yesterday I travelled one third of the remaining distance.
I now have 21 miles left to travel.
How far is it from Birmingham to Glasgow in total?
Puzzle Copyright © Kevin Stone
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Hint
How many miles were left at the start of yesterday?
Answer
336 miles.
Reasoning
To work these numbers out, it is perhaps easier to start at the end and work backwards.
Yesterday I travelled 1⁄3 of the distance to leave 21 miles, therefore there were 31.5 miles (=21 ÷ 2⁄3) at the start of yesterday.
On day 3 I travelled 3⁄4 of the distance to leave 31.5 miles, therefore there were 126 miles (=31.5 ÷ 1⁄4) at the start of day 3.
On day 2 I travelled 1⁄2 of the distance to leave 126 miles, therefore there were 252 miles (=126 ÷ 1⁄2) at the start of day 2.
On day 1, I travelled 1⁄4 of the distance to leave 252 miles, therefore there were 336 miles (=252 ÷ 3⁄4) at the start of day 1.
Double-Checking
On day one, I travelled one quarter of the distance (84, leaving 252).
On day two, I travelled one half of the remaining distance (126, leaving 126).
On day three, I travelled three quarters of the remaining distance (94.5, leaving 31.5).
Yesterday I travelled one third of the remaining distance (10.5).
I now have 21 miles left to travel.
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