Aaah, now I see what you've done, you've divided each length by the tile side measurement then multiplied the two together.
Nevetsnosnas, working out an area is really easy especially if the area is square or rectangular. All you need to do is multiply the length of one side (in your case 16ft) by the other (5ft10). However in your question you've given two different units (feet and inches) and metric (500mm). So the only way to work it out is to convert both to metric or imperial. I'll take you through it step by step.
When I'm working out measurements, I take each foot as being 0.3m (there's about 305mm in a foot so this is close enough for building materials). So, you have 16 feet on one side, to convert to metres just multiply 16 x 0.3 = 4.8m.
The other side is 5 feet 10 inches, so 5 x 0.3 = 1.5m, 10 inches works out at 0.25m (25mm per inch) so you have a second side of 1.75m (1.5 + 0.25).
Now multiply the two together, 4.8 x 1.75 = 8.4m2 and you have an area of square metres to fill (imagine an area of one square metre, you have the equivalent of just under 8.5 of these laid side by side).
Each tile covers an area of 0.25m2, once again this is found by multiplying the two sides together, 0.5 x 0.5 = 0.25m2.
You now divide your 8.4m2 by 0.25m2, (8.4 / 0.25 = 33.6).
You need a minimum of 34 tiles, add at least 5% for wasteage and breakages so 40 should be enough.
Hope this helps and hasn't confused you more!