## September 2005

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In my September diary I posed the following brain-teaser, sent in by Boris Zeldovich.

Two old ladies started their journeys simultaneously, exactly at the dawn, along the same trail, towards each other's village. Moving with constant, but different, speeds, (pace ? I am translating from Russian. BZ.) they met each other at 12:00 sharp (noon). Without stopping to chat, they continued their steady motions. One lady reached her destination, i.e. other lady's starting point, at 4:00 PM, while the other reached her destination at 9:00 PM. Question. At what time the dawn happened (cracked ? BZ) this day?

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*Solution*

A diagram helps. The vertical coordinate z is the distance counted from the place of
departure of, e.g., the second
old lady, while the horizontal coordinate is time. The speeds of the babushkas being constant, their trajectories on
this diagram are straight
lines, albeit drawn at somewhat different angles with the axes. The time elapsed from the dawn to the moment of the
ladies' meeting at noon is
denoted by *t*, and *a* = 4 hours and *b* = 9 hours denote the times spent to
reach the corresponding
destinations after the meeting. Since upper and lower *t*-axes on this diagram are parallel, the resultant
triangles are similar. By some
theorem or other, the heights (dashed lines) in these similar triangles divide the corresponding sides of the triangles
in a proportional ratio. In
the notation introduced above, *t* / *a* = *b* / *t*. So
*t*^{ 2} =*ab*, whence *t* =6. Counting 6 hours back from noon, one gets
the time of dawn as
6:00am.