jmc

Let $r$ be the remaining student's score. We know that the average, 70, is equal to $\frac{70+80+90+r}{4}$. We can use basic algebra to solve for $r$: $$\frac{70+80+90+r}{4}=70$$ $$\frac{240+r}{4}=70$$ $$240+r=280$$ $$r=40$$ giving us the answer of $\boxed{40}$.