Angles in Polygons (Edexcel IGCSE Maths A (Modular): Foundation Unit 2): Exam Questions

The diagram shows two congruent regular pentagons drawn inside a regular octagon.

One side of each pentagon lies along a side of the octagon.

$AB$ is a side of the octagon.
$AC$ is a side of one of the pentagons.
$BC$ is a side of the other pentagon.

Work out the size of angle $y$.
Show your working clearly