3. Evaluate \( \frac{m}{n t}+(-t)-\frac{n}{t} \) if \( \mathrm{m}=20, \mathrm{n}=4 \) and \( \mathrm{t}=-1 \)

Let's plug in the values \( m = 20 \), \( n = 4 \), and \( t = -1 \) into the expression \( \frac{m}{n t} + (-t) - \frac{n}{t} \). First, calculate \( n t \): \[ n t = 4 \times (-1) = -4 \] Now, calculate \( \frac{m}{n t} \): \[ \frac{m}{n t} = \frac{20}{-4} = -5 \] Next, calculate \( -t \): \[ -t = -(-1) = 1 \] Now, calculate \( \frac{n}{t} \): \[ \frac{n}{t} = \frac{4}{-1} = -4 \] Now combine all the parts: \[ \frac{m}{n t} + (-t) - \frac{n}{t} = -5 + 1 - (-4) \] Simplifying, we have: \[ -5 + 1 + 4 = 0 \] Therefore, the final evaluation is: \[ \boxed{0} \]