Let the lengths of the two equal sides of the isosceles triangle be \( x \). The base is given as 20 centimeters, so the perimeter can be expressed as:
\[ x + x + 20 = 76 \]
This simplifies to:
\[ 2x + 20 = 76 \]
Subtracting 20 from both sides, we get:
\[ 2x = 56 \]
Now, dividing both sides by 2:
\[ x = 28 \]
Thus, each of the equal sides of the isosceles triangle is 28 centimeters long.
The answer is:
28