Concept:
Two oscillations come in phase again when their phase difference becomes multiple of \(2\pi\), i.e., time equals LCM of periods.
Step 1: Given periods.
\[
T_1 = T, \quad T_2 = \frac{5T}{4}
\]
Step 2: Find common time.
We need smallest \(t\) such that:
\[
\frac{t}{T} = n, \quad \frac{t}{5T/4} = m
\]
Step 3: Solve using LCM.
LCM of \(T\) and \(5T/4\):
\[
= 5T
\]
Step 4: Conclusion.
\[
\boxed{5T}
\]