To simplify the expression \( t^{400} \cdot t^{100} \) using the Quotient Rule of Integer Exponents, we apply the property of exponents that states:
\[ a^m \cdot a^n = a^{m+n} \]
In this case, \( m = 400 \) and \( n = 100 \):
\[ t^{400} \cdot t^{100} = t^{400 + 100} = t^{500} \]
Therefore, the simplified expression is:
\[ t^{500} \]