If you call [tex] m [/tex] the mass of the ant and [tex] l [/tex] the load, we have the equation
[tex] m = \cfrac{l}{10} [/tex]
In fact, the mass of the ant is one tenth of the load, which is exactly what this equation states.
Since we are given the load, we simply need to plug its value in the equation to deduce the mass of the ant:
[tex] m = \cfrac{\frac{7}{250}}{10} = \cfrac{7}{250}\cdot\cfrac{1}{10} = \cfrac{7}{2500} [/tex]
