Number of distinct terms in \((a+b+c)^{n}\) = number of non-negative integer solutions to:
\[
x + y + z = n
\]
where \(x, y, z\) are exponents of \(a, b, c\).
Formula: \(\binom{n+3-1}{3-1} = \binom{n+2}{2}\).
For \(n = 20\):
\[
\binom{22}{2} = 231
\]