Update learning opportunities

app/commands/user/challenges/featured_exercises_progress_48_in_24.rb

@@ -13,10 +13,10 @@ class User::Challenges::FeaturedExercisesProgress48In24
     { week: 6, slug: 'list-ops', title: 'List Ops', featured_tracks: %w[gleam swift sml], learning_opportunity: "This exercise is the perfect space to explore the imperative and functional approaches to common functions. Compare both types of coding, and try to optimise your solution to use tail-call recursion!" },
     { week: 7, slug: 'acronym', title: 'Acronym', featured_tracks: %w[haskell tcl powershell], learning_opportunity: "This exercise is the perfect space to explore regular expressions. You can solve the exercise using basic techniques or more complex ones, but can you solve it using just one regular single expression?" },
     { week: 8, slug: 'circular-buffer', title: 'Circular Buffer', featured_tracks: %w[c groovy scala], learning_opportunity: "Circular Buffers are First In, First Out (FIFO) queues that are a fixed size. They're usually backed by an array, which gives it great spatial/data locality (good for performance!). Explore different ways to create them yourself!" },
-    { week: 9, slug: 'parallel-letter-frequency', title: 'Parallel Letter Frequency', featured_tracks: %w[go java elixir], learning_opportunity: "" },
+    { week: 9, slug: 'parallel-letter-frequency', title: 'Parallel Letter Frequency', featured_tracks: %w[go java elixir], learning_opportunity: "Learn about the difference between concurrency and parallellism and how they affect performance. Explore different ways to run code in parallel and how to avoid concurrency issues through mutexes or concurrent collections." },
     { week: 10, slug: 'allergies', title: 'Allergies', featured_tracks: %w[nim elm rust], learning_opportunity: "Learn about bitwise operations, bitshifting, bitwise comparison and other binary number concepts, or try solving the exercise functionally with pattern matching and recursion." },
-    { week: 11, slug: 'sieve', title: 'Sieve', featured_tracks: %w[zig bash fortran], learning_opportunity: "" },
-    { week: 12, slug: 'luhn', title: 'Luhn', featured_tracks: %w[typescript perl5 awk], learning_opportunity: "" },
+    { week: 11, slug: 'sieve', title: 'Sieve', featured_tracks: %w[zig bash fortran], learning_opportunity: "Though usually implemented using a fixed-size buffer and nested loops, this exercise is the perfect place to explore bitwise representations and clever loop limits to improve memory and performance." },
+    { week: 12, slug: 'luhn', title: 'Luhn', featured_tracks: %w[typescript perl5 awk], learning_opportunity: "Explore different ways to validate input and to double every other digit, which can be done is some surprising ways, with imperative, OOP and functional ways all possible!" },
     { week: 13, slug: 'scrabble-score', title: 'Scrabble Score', featured_tracks: %w[python scheme c], learning_opportunity: "" },
     { week: 14, slug: 'difference-of-squares', title: 'Difference Of Squares', featured_tracks: %w[r wasm swift], learning_opportunity: "" },
     { week: 15, slug: 'pangram', title: 'Pangram', featured_tracks: %w[julia elixir go], learning_opportunity: "" },