GayLussac's Law  

Or
P_{i}T_{f} = P_{f}T_{i}
Where,
P_{i} = initial pressure
T_{i} = initial temperature
P_{f} = final pressure
T_{f}= final temperature
Example:
Calculate and find the temperature of the gas which has a pressure of 25 kPa.
Initial Pressure (Pi) = 25 kPa
Initial Temperature (Ti) = 5 K
Final Pressure (Pf) = 15 kPa
Solution:
Apply Formula:
P_{i}/T_{i} = P_{f}/T_{f}
Final Temperature (Tf) = 3K