jmc

We should notice that we can turn the information we are given into a linear equation and just solve for our set variables. I'll use the variables $x$ and $y$ for the scores on the last two tests. $$\frac{76+94+87+x+y}{5}=81,$$ $$\frac{257+x+y}{5}=81.$$ We can now cross multiply to get rid of the denominator. $$257+x+y=405,$$ $$x+y=148.$$ Now that we have this equation, we will assign $y$ as the lowest score of the two other tests, and so: $$x=100,$$ $$y=48.$$ Now we know that the lowest score on the two other tests is $\boxed{48}$.