2 files changed, 54 insertions, 0 deletions
diff --git a/challenge-147/feng-chang/raku/ch-1.raku b/challenge-147/feng-chang/raku/ch-1.raku
new file mode 100755
index 0000000000..d3dab98fae
--- /dev/null
+++ b/challenge-147/feng-chang/raku/ch-1.raku
@@ -0,0 +1,24 @@
+#!/bin/env raku
+
+unit sub MAIN(UInt:D \num = 20);
+
+my @lefty-primes = 2, 3, 5, 7;
+my ($start, $end) = 0, 3;
+
+my UInt $sz  = @lefty-primes.elems;
+
+while $sz < num {
+    for 1..9 -> \d {
+        for @lefty-primes[$start..$end] -> \n {
+            my \m = (d ~ n).UInt;
+            if m.is-prime {
+                @lefty-primes.push(m);
+                ++$sz;
+            }
+        }
+    }
+    ($start, $end) = $end + 1, $sz - 1;
+    last if $end < $start;
+}
+
+put @lefty-primes[^((num,$sz).min)];
diff --git a/challenge-147/feng-chang/raku/ch-2.raku b/challenge-147/feng-chang/raku/ch-2.raku
new file mode 100755
index 0000000000..b2ac1fdad3
--- /dev/null
+++ b/challenge-147/feng-chang/raku/ch-2.raku
@@ -0,0 +1,30 @@
+#!/bin/env raku
+
+# https://www.mathblog.dk/project-euler-44-smallest-pair-pentagonal-numbers/
+# is-pentagonal() is key to speed it up
+
+my UInt $i = 1;
+ 
+outer-loop:
+loop {
+    ++$i;
+    my UInt \n = pentagon($i);
+
+    loop (my UInt $j = $i - 1; $j > 0; --$j) {
+        my UInt \m = pentagon($j);
+        if is-pentagonal(n - m) && is-pentagonal(n + m) {
+            put "\n", n, ' ', m;
+            last outer-loop;
+        }
+    }
+    print "{$i}\r";
+}
+
+sub pentagon(UInt:D \n --> UInt:D) {
+    n * (3 * n - 1) div 2
+}
+
+sub is-pentagonal(UInt:D \n --> Bool:D) {
+    my \p = (sqrt(1.0 + 24 * n) + 1.0) / 6.0;
+    p == p.UInt
+}