A point is plotted on a Mercator projection within China. Calculate the longitude and latitude of the point exactly on the opposite side of the earth (i.e. antipodal) to that point. It is the point from which you could dig stratight down to reach China.

Are you saying you don't know the point's lat or long either? A Mercator is labeled with lat and long, pretty much a wothless map I might add.

Longs' and Lats' are valid no matter what map used. Mercators merely distort things a bit. Probably being used in this exercise because it DOES distort things - making it hard to fudge the answer.

I believe the point is that merong has been given the location in China and needs to know what is directly opposite. On a sphere, points directly opposite are 180 degrees away from each other.

Look at this:Lat - Long Finder: This page helps you find Latitude and Longitude (http://www.satsig.net/maps/lat-long-finder.htm)