Answer: The largest angle would be the angle connecting the 10m and the 12m sides. After drawing a "proportionate" triangle, one would see that the angle on the 10m and 20m sides is very small, and the angle connecting the 12m and 20m sides is even smaller. the 10m and 12m angle needs to be large enough to accommodate the 20m side, making it the largest angle in the triangle.
Explanation: