diff --git a/readme.md b/readme.md
index ff7a266..dc4b834 100644
--- a/readme.md
+++ b/readme.md
@@ -9,7 +9,7 @@ All of the solutions here are written in rust. [main.rs](src/main.rs) is the hom
 
 ## Challenge Completion
 
-### <!-- completed -->14<!-- completed --> out of 100 public challenges completed.
+### <!-- completed -->15<!-- completed --> out of 100 public challenges completed.
 
 - [x] 1 - [Multiples of 3 or 5](src/bin/1.rs)
 - [x] 2 - [Even Fibonacci numbers](src/bin/2.rs)
@@ -25,7 +25,7 @@ All of the solutions here are written in rust. [main.rs](src/main.rs) is the hom
 - [x] 12 - [Highly divisible triangular number](src/bin/12.rs)
 - [x] 13 - [Large sum](src/bin/13.rs)
 - [x] 14 - [Longest Collatz sequence](src/bin/14.rs)
-- [ ] 15 - Lattice paths
+- [x] 15 - [Lattice paths](src/bin/15.rs)
 - [ ] 16 - Power digit sum
 - [ ] 17 - Number letter counts
 - [ ] 18 - Maximum path sum I
diff --git a/src/bin/15.rs b/src/bin/15.rs
new file mode 100644
index 0000000..b005fe1
--- /dev/null
+++ b/src/bin/15.rs
@@ -0,0 +1,43 @@
+/*
+Problem 15 - Lattice paths
+
+Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.
+How many such routes are there through a 20×20 grid?
+*/
+
+fn binomial(n: usize, k: usize) -> Result<usize, &'static str> {
+    if k > n {
+        return Err("k must be less than n!");
+    } else if k == 0 {
+        return Ok(1);
+    } else if k > n / 2 {
+        return binomial(n, n - k);
+    }
+
+    return Ok(n * binomial(n - 1, k - 1).unwrap() / k);
+}
+
+fn count_lattice_paths(width: usize, height: usize) -> usize {
+    // All paths have width + height segments. k can either be width or height
+    // (symmetry of binomial coefficients)
+    if width != height {
+        return binomial(width + height, width).unwrap();
+    }
+    
+    // (2n, n)
+    else {
+        let mut result: usize = 1;
+
+        for i in 1..width {
+            result *= (width + i) / i;
+        }
+
+        return result;
+    }
+}
+
+fn main() {
+    let routes = count_lattice_paths(20, 20);
+
+    println!("The amount of routes through a 20x20 grid is {routes}");
+}