another simple maths question

Hey peeps

Every frame, im moving an entity in a certain direction by speed "speed"...
Im also storing how much it moved by last time, to add a drifting effect (whats it called?)

Now, im having a problem when detecting my object has reached its target: without the drift stuff, i can just go

if entitydistance(object,target)<speed then reached=1

but, How do i do it including the old forces?

Basically, i need to figure out:

nspeed=0
for o=1 to 10
nspeed=nspeed+speed
speed=speed*.9
next
print speed

by maths. i know this is only aproximate but itl be enough... if you know of a better way, post also? :D

cheers peeps.