Radio Telescope Resolution - Centaurus A

[Timiop2008]

Problem:
Centaurus A is 15 million light years away and a radio telescope can resolve structures of 1s of arc across. Show that the Structure is 75 light years across.
Please Help!

 

[mgb_phys]

1 sec of arc is 1 / 60 * 1/60 of a degree.
Imagine a circle 15M light years in radius with you at the centre, what length of the circumference would 1arcsec measure?