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+#+SETUPFILE: ~/.emacs.d/org-templates/level-2.org
+#+HTML_LINK_UP: ../index.html
+#+OPTIONS: toc:2
+#+EXPORT_FILE_NAME: index
+#+DATE: 2020-12-08
+#+TITLE: Challenge 090
+
+* Task 2 - Ethiopian Multiplication
+You are given two positive numbers $A and $B.
+
+Write a script to demonstrate [[https://threesixty360.wordpress.com/2009/06/09/ethiopian-multiplication/][Ethiopian Multiplication]] using the given
+numbers.
+** Raku
+- Program: [[file:perl/ch-2.raku]]
+
+Start by taking =$A= & =$B= which are defined to be =Int= & positive.
+#+BEGIN_SRC raku
+sub MAIN (
+    #= positive numbers
+    Int $A is copy where * > 0,
+    Int $B is copy where * > 0
+) {
+    ...
+}
+#+END_SRC
+
+Here's relevant part from the link that was given above:
+#+BEGIN_QUOTE
+Start with the two numbers on top. Halve one, ignoring any remainders or
+fractions, and double the other, stopping when you get to 1.
+
+14 & 12
+7 & 24
+3 & 48  [See how I ignored the fact that halving 7 leaves 1 left over?]
+1 & 96  <— Stop here.
+
+Now look at the numbers on the right. Some are across from an even
+number: in this case, 12 is across from the original 14. Ignore those,
+and add the rest. So we’ll add 24, 48, and 96, which were across from
+odd numbers, and get 168. And that’s the product! Isn’t that cool?
+#+END_QUOTE
+
+We do the same thing & also print the instructions.
+#+BEGIN_SRC raku
+my %sets;
+
+say "Ethopian Multiplication.\n";
+say "Start with $A, $B.";
+say "Divide $A by 2 & multiple $B by 2 at each step.";
+say "Continue until $A equals 1. Drop the remainder, both should be Integer.\n";
+
+say "$A, $B";
+while True {
+    %sets{$A} = $B.Int;
+    $A = ($A / 2).Int;
+    $B = ($B * 2).Int;
+    last if $A < 1;
+    say "$A, $B";
+}
+
+say "\nNow to find the product, simply add all the numbers on right side of ','.";
+say "But skip those numbers which have an even number on the left side.\n";
+
+my Int $product = 0;
+for %sets.sort({.key.Int}).reverse -> $pair {
+    if $pair.key % 2 != 0 {
+        $product += $pair.value;
+        say "- Adding ", $pair.value, " to product.";
+    } else {
+        say "- Skipping ", $pair.value, " because ", $pair.key, " is even.";
+    }
+}
+
+say "\nProduct: $product";
+#+END_SRC