Concept:
Using ideal gas law:
\[
PV = nRT
\]
Step 1: Initial moles.
For identical cylinders (same \(V, T\)):
\[
n_1 = \frac{P_1 V}{RT}, \quad n_2 = \frac{P_2 V}{RT}
\]
Step 2: Total moles.
\[
n_{\text{total}} = \frac{(P_1 + P_2)V}{RT}
\]
Step 3: Final pressure.
Now gases are in one cylinder (volume \(V\)):
\[
P = \frac{n_{\text{total}} RT}{V} = P_1 + P_2
\]
\[
P = 2.5 + 1 = 3.5 \, \text{atm}
\]