What is the minimum number of equilateral triangles, of side length 1 unit, needed to cover an equilateral triangle of side length 10 units?
The ratio of the sides of the small to big equilateral triangle (note that they are similar) is $1/10$, so the ratio of their areas is $(1/10)^2 = 1/100$. So the big equilateral triangle has 100 times the area of a small one, so it will take $\boxed{100}$ small triangles to cover the big one.