diff --git a/code/beginner/tutorial7-instancing/src/challenge.rs b/code/beginner/tutorial7-instancing/src/challenge.rs
index 43a4f86c..c31d936b 100644
--- a/code/beginner/tutorial7-instancing/src/challenge.rs
+++ b/code/beginner/tutorial7-instancing/src/challenge.rs
@@ -286,34 +286,6 @@ struct State {
     instance_buffer: wgpu::Buffer,
 }
 
-fn quat_mul(q: cgmath::Quaternion<f32>, r: cgmath::Quaternion<f32>) -> cgmath::Quaternion<f32> {
-    // This block uses quaternions of the form of
-
-    // q=q0+iq1+jq2+kq3
-
-    // and
-
-    // r=r0+ir1+jr2+kr3.
-
-    // The quaternion product has the form of
-
-    // t=q×r=t0+it1+jt2+kt3,
-
-    // where
-
-    // t0=(r0 q0 − r1 q1 − r2 q2 − r3 q3)
-    // t1=(r0 q1 + r1 q0 − r2 q3 + r3 q2)
-    // t2=(r0 q2 + r1 q3 + r2 q0 − r3 q1)
-    // t3=(r0 q3 − r1 q2 + r2 q1 + r3 q0
-
-    let w = r.s * q.s - r.v.x * q.v.x - r.v.y * q.v.y - r.v.z * q.v.z;
-    let xi = r.s * q.v.x + r.v.x * q.s - r.v.y * q.v.z + r.v.z * q.v.y;
-    let yj = r.s * q.v.y + r.v.x * q.v.z + r.v.y * q.s - r.v.z * q.v.x;
-    let zk = r.s * q.v.z - r.v.x * q.v.y + r.v.y * q.v.x + r.v.z * q.s;
-
-    cgmath::Quaternion::new(w, xi, yj, zk)
-}
-
 impl State {
     async fn new(window: &Window) -> Self {
         let size = window.inner_size();
@@ -592,7 +564,7 @@ impl State {
         for instance in &mut self.instances {
             let amount = cgmath::Quaternion::from_angle_y(cgmath::Rad(ROTATION_SPEED));
             let current = instance.rotation;
-            instance.rotation = quat_mul(amount, current);
+            instance.rotation = amount * current;
         }
         let instance_data = self
             .instances