Update physics_introduction.rst

Corrected decimal calculation for layers

classes/class_clippedcamera.rst

@@ -218,3 +218,4 @@ Removes a collision exception with the specified :ref:`RID<class_RID>`.
 Sets the specified bit index to the ``value``.
 
 **Note:** Bit indices range from 0-19.
+

classes/class_raycast.rst

@@ -290,3 +290,4 @@ Removes a collision exception so the ray does report collisions with the specifi
 Sets the bit index passed to the ``value`` passed.
 
 **Note:** Bit indexes range from 0-19.
+

classes/class_raycast2d.rst

@@ -284,3 +284,4 @@ Removes a collision exception so the ray does report collisions with the specifi
 - void **set_collision_mask_bit** **(** :ref:`int<class_int>` bit, :ref:`bool<class_bool>` value **)**
 
 Sets or clears individual bits on the collision mask. This makes selecting the areas scanned easier.
+

classes/class_tilemap.rst

@@ -690,3 +690,4 @@ Updates the tile map's quadrants, allowing things such as navigation and collisi
 - :ref:`Vector2<class_Vector2>` **world_to_map** **(** :ref:`Vector2<class_Vector2>` world_position **)** const
 
 Returns the tilemap (grid-based) coordinates corresponding to the given local position.
+

tutorials/physics/physics_introduction.rst

@@ -163,8 +163,9 @@ would be as follows:
     0x000d
     # (This value can be shortened to 0xd)
 
-    # Decimal - Raise each layer to be enabled (1, 3, and 4) to the power of 2 and add all the results together.
-    (1^2) + (3^2) + (4^2) = 13
+    # Decimal - Add the results of 2 to the power of (layer be enabled-1).
+    # (2^(1-1)) + (2^(3-1)) + (2^(4-1)) = 1 + 4 + 8 = 13
+    pow(2, 1) + pow(2, 3) + pow(2, 4)
 
 
 Area2D