Chest

"They all dressed the same way, pleated trousers, and tight shirts that accentuated their arms and chest."

Why the singular 'chest'?

I don't know but it's the same in BCS.

Maybe the "arms" refers to only one guys arms, so chest refers to his chest.

Hoo, I know. But why?

<<"They all dressed the same way, pleated trousers, and tight shirts that accentuated their arms and chest." >>

Because think of it this way:

"They all dressed the same way, pleated trousers, and tight shirts that accentuated each one's arms and chest." --each individual only has one chest.

It would buse plural "chests" if you speciaief all of them:
"They all dressed the same way, pleated trousers, and tight shirts that accentuated all of their arms and chests."

<<speciaief >>

That should be "specified"