Hi Speedbird,
Its been a while since I did one of these but, perhaps something like this?
Your at 8000ft at 20miles and you need to be at 1000ft at 5 miles.
This means we need to lose 7000ft in 15 miles.
At 150kts, it will take you 6 minutes to fly those 15 miles 15/150 x60 =6 mins
So, we have 7000ft to lose in 6 minutes, 7000/6 = 1166ft, or about 1200ft/min if I was flying :-)
It often helped me to draw a diagram out and look at the problem side on.
Hope that helps,
Regards,
Buster
Re Alex, I had assumed (never do that! ) that airspeed and groundspeed were the same given no wind information. That makes it a little more straightforward.