Sandra's age is given by the expression 80 - 40 / 10 x 4.

$$80 - 16 = 64$$

Sandra's age is $\boxed{64 \text{ years old}}$.

2)
Given the equations:

1. $m^2 + n^2 = 200$
2. $m + n = 200$

$$(m + n)^2 = (200)^2$$ $$m^2 + 2mn + n^2 = 40000$$

$$(m^2 + n^2) + 2mn = 40000$$ $$200 + 2mn = 40000$$

$$2mn = 40000 - 200$$ $$2mn = 39800$$ $$mn = \frac{39800}{2}$$ $$mn = 19900$$

This equation is typically written as $x^2 - (m+n)x + mn = 0$.
Substitute the values $m+n=200$ and $mn=19900$:

$$x^2 - 200x + 19900 = 0$$

Here, $a=1$, $b=-200$, $c=19900$.

$$\Delta = (-200)^2 - 4(1)(19900)$$ $$\Delta = 40000 - 79600$$ $$\Delta = -39600$$

Since the discriminant $\Delta$ is negative ($\Delta < 0$), there are no real solutions for $x$. This means there are no real numbers $m$ and $n$ that can satisfy both given equations simultaneously.
Therefore, the ratio $\frac{n}{m}$ cannot be determined as a real number.

The problem has $\boxed{\text{no real solution}}$.