It should be fairly obvious from the replies above, for a no wind day - but wind will completely change the situation - hence the infamous 'whizz wheel' where you can graphically add the wind vector and then derive groundspeed.
Or you could ball park it with the wind direction degrees of beam, i use the "6" method ie. if the wind is 30 degrees of beam its 3/6 (half) the wind speed decducted from the GS, if its anything more than 60 degrees (6, 6's) deduct the full wind speed.
If I have this wrong I will gladly be corrected.