To find the part of the pizza that was not eaten, you can subtract the total amount eaten by Randal and Griffin from the whole pizza (which is represented as 1).
First, convert Griffin's portion (0.5) into a fraction that has the same denominator as Randal's portion (3/8).
0.5 is equivalent to 4/8 in fractional form.
Now, add the portions eaten by Randal and Griffin:
\[ \frac{3}{8} + \frac{4}{8} = \frac{3 + 4}{8} = \frac{7}{8} \]
Now, subtract the total portion eaten from the whole pizza:
\[ 1 - \frac{7}{8} = \frac{8}{8} - \frac{7}{8} = \frac{1}{8} \]
Thus, the part of the pizza that was not eaten is \( \frac{1}{8} \).