Momentum conservation? stranded in space push on something to get to ship

[awertag]

Homework Statement

You have a 99 kg mass and are stranded away from your ship, at rest, next to a giant 1800 kg ball of space doody. You push on the doody giving it a speed of 0.11 m/s directly away from the ship. Seven-and-a-half seconds later you come into contact with your ship. What was the initial distance of separation between you and your ship?

Homework Equations

The Attempt at a Solution

i tried doing it as an explosion problem with momentum conservation:
0=m1v1f + m2v2f
0=(99)(v1f)+(1800)(-11)
v1f=200 m/s

then i did d=vt to get d...got 750 metres, was wrong? help please!

 

[Sourabh N]

d = v*t = 200*7.5 = 1500 m.

 

[awertag]

haha right i know i tried that first. then i did average v = 100

 

[Sourabh N]

Why?

 

[awertag]

i don't know...because 1500 wasnt right so i just tried something else