[euler] add docs for solution to problem 151

categories/euler/prob151-shlomif.p6

@@ -1,5 +1,40 @@
 use v6;
 
+=begin pod
+
+=TITLE Paper sheets of standard sizes: an expected-value problem
+
+=AUTHOR Shlomi Fish
+
+L<https://projecteuler.net/problem=151>
+
+A printing shop runs 16 batches (jobs) every week and each batch requires a
+sheet of special colour-proofing paper of size A5.
+
+Every Monday morning, the foreman opens a new envelope, containing a large
+sheet of the special paper with size A1.
+
+He proceeds to cut it in half, thus getting two sheets of size A2. Then he
+cuts one of them in half to get two sheets of size A3 and so on until he
+obtains the A5-size sheet needed for the first batch of the week.
+
+All the unused sheets are placed back in the envelope.
+
+At the beginning of each subsequent batch, he takes from the envelope one
+sheet of paper at random. If it is of size A5, he uses it. If it is larger,
+he repeats the 'cut-in-half' procedure until he has what he needs and any
+remaining sheets are always placed back in the envelope.
+
+Excluding the first and last batch of the week, find the expected number of
+times (during each week) that the foreman finds a single sheet of paper in
+the envelope.
+
+Give your answer rounded to six decimal places using the format x.xxxxxx.
+
+Expected result: 0.464398
+
+=end pod
+
 my $sum = 0;
 
 # rec is short for "recurse".
@@ -36,3 +71,5 @@ sub rec($n, $d, @counts, $result)
 rec(1, 1, [1],0);
 
 say "Result == ", ($sum - 2).fmt("%.6f");
+
+# vim: expandtab shiftwidth=4 ft=perl6