How much Food do we have?

Me and Mystery Man have been engulfed in a math project to find out how much of the earth we have to grow food. Two Waikato University students from SIFE (students in free enterprise) came in and said we only had 1/32 of the earth's surface to grow our crop. So we set out to prove if this was true. First we got the Earths land and sea surface area and converted it to rough fractions. Using my talent i determined that there were:
surface area: 510 065 600 km²
land area: 148 330 000 km²
sea area: 361 740 000 km²
using my further talent i determined that this meant that:
15/51 surface area is land.
36/51 surface area is ocean.
This instantly eliminated 36/51 from our equasion.
with the 15/51 we had left, we had to take out the mountanious, desert, forest, and urban (buildings) land.
In the end we had about 1.5/51(something like that) left to grow food. We then found out this was roughly equivelant to 1/33( includes permanent crop and arable land). So, in conclusion, we have about 1/33 of the earths surface area to grow food. So the two university students were incorrect by just a tiny bit.
This is important because we have to be sustainable, and be careful about what we do with the tiny bit of land we have for growing food.

This post is also on my sustainability blog because i didn't know where to put it.