A pentagon has four interior angles each equal to $110^o$. Find the degree measure of the fifth interior angle.
1. **Determine the sum of the interior angles of a pentagon:**
   The formula for the sum of the interior angles of a polygon with \( n \) sides is given by:
   \[
   \text{Sum of interior angles} = 180^\circ \times (n - 2)
   \]
   For a pentagon (\( n = 5 \)):
   \[
   \text{Sum of interior angles} = 180^\circ \times (5 - 2) = 180^\circ \times 3 = 540^\circ
   \]

2. **Calculate the sum of the four given interior angles:**
   Each of the four given interior angles is \( 110^\circ \). Therefore, the sum of these four angles is:
   \[
   4 \times 110^\circ = 440^\circ
   \]

3. **Find the measure of the fifth interior angle:**
   The sum of all five interior angles must equal the total sum of the interior angles of the pentagon. Let \( x \) be the measure of the fifth interior angle. Then:
   \[
   440^\circ + x = 540^\circ
   \]
   Solving for \( x \):
   \[
   x = 540^\circ - 440^\circ = 100^\circ
   \]

The final answer is \( \boxed{100^\circ} \).