diff --git a/crates/python_api/src/sfpe_handbook/chapter_50/equation_50_17.rs b/crates/python_api/src/sfpe_handbook/chapter_50/equation_50_17.rs
index 3a72c4f..3fe7135 100644
--- a/crates/python_api/src/sfpe_handbook/chapter_50/equation_50_17.rs
+++ b/crates/python_api/src/sfpe_handbook/chapter_50/equation_50_17.rs
@@ -43,4 +43,4 @@ fn stairwell_temperature(t_0: f64, eta: f64, t_b: f64) -> PyResult<f64> {
 pub fn equation_50_17(m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_function(wrap_pyfunction!(stairwell_temperature, m)?)?;
     Ok(())
-}
\ No newline at end of file
+}
diff --git a/crates/python_api/src/sfpe_handbook/chapter_50/equation_50_20.rs b/crates/python_api/src/sfpe_handbook/chapter_50/equation_50_20.rs
index 2f2c6ba..f28b274 100644
--- a/crates/python_api/src/sfpe_handbook/chapter_50/equation_50_20.rs
+++ b/crates/python_api/src/sfpe_handbook/chapter_50/equation_50_20.rs
@@ -43,4 +43,4 @@ fn visibility(k: f64, l: f64, lambda: f64) -> PyResult<f64> {
 pub fn equation_50_20(m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_function(wrap_pyfunction!(visibility, m)?)?;
     Ok(())
-}
\ No newline at end of file
+}
diff --git a/crates/sfpe_handbook/src/chapter_50/equation_50_16.rs b/crates/sfpe_handbook/src/chapter_50/equation_50_16.rs
index b0a0a9f..7f839be 100644
--- a/crates/sfpe_handbook/src/chapter_50/equation_50_16.rs
+++ b/crates/sfpe_handbook/src/chapter_50/equation_50_16.rs
@@ -28,4 +28,4 @@ mod tests {
         let expected = 2.035971223;
         assert!((result - expected).abs() < 1e-6);
     }
-}
\ No newline at end of file
+}
diff --git a/crates/sfpe_handbook/src/chapter_50/equation_50_17.rs b/crates/sfpe_handbook/src/chapter_50/equation_50_17.rs
index 79c8523..3b58b4d 100644
--- a/crates/sfpe_handbook/src/chapter_50/equation_50_17.rs
+++ b/crates/sfpe_handbook/src/chapter_50/equation_50_17.rs
@@ -3,7 +3,12 @@ pub fn stairwell_temperature(t_0: f64, eta: f64, t_b: f64) -> f64 {
 }
 
 #[cfg(not(coverage))]
-pub fn stairwell_temperature_equation(t_s: String, t_0: String, eta: String, t_b: String) -> String {
+pub fn stairwell_temperature_equation(
+    t_s: String,
+    t_0: String,
+    eta: String,
+    t_b: String,
+) -> String {
     format!("{} = {} + {} \\times ( {} - {} )", t_s, t_0, eta, t_b, t_0)
 }
 
diff --git a/crates/sfpe_handbook/src/chapter_59.rs b/crates/sfpe_handbook/src/chapter_59.rs
new file mode 100644
index 0000000..bc3c565
--- /dev/null
+++ b/crates/sfpe_handbook/src/chapter_59.rs
@@ -0,0 +1 @@
+pub mod equation_59_4;
diff --git a/crates/sfpe_handbook/src/chapter_59/equation_59_4.rs b/crates/sfpe_handbook/src/chapter_59/equation_59_4.rs
new file mode 100644
index 0000000..3a27512
--- /dev/null
+++ b/crates/sfpe_handbook/src/chapter_59/equation_59_4.rs
@@ -0,0 +1,20 @@
+pub fn speed(k: f64, a: f64, d: f64) -> f64 {
+    k - a * k * d
+}
+
+#[cfg(not(coverage))]
+pub fn speed_equation(s: String, k: String, a: String, d: String) -> String {
+    format!("{} = {} - {} \\cdot {} \\cdot {}", s, k, a, k, d)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_speed() {
+        let result = speed(1.4, 0.266, 2.0);
+        let expected = 0.6552;
+        assert!((result - expected).abs() < 1e-6);
+    }
+}
diff --git a/crates/sfpe_handbook/src/lib.rs b/crates/sfpe_handbook/src/lib.rs
index 3ca7985..59930f3 100644
--- a/crates/sfpe_handbook/src/lib.rs
+++ b/crates/sfpe_handbook/src/lib.rs
@@ -1,2 +1,3 @@
 pub mod chapter_14;
 pub mod chapter_50;
+pub mod chapter_59;