At the temperature of 30degree C and pressure of 1*10^5 pa, 30dm3 of H2 gas is burned with 30dm3 of O2 gas in a container of 60dm3. Calculate the pressure different after the complete combustion and cooling upto 30degree C again.

Won't that be just 1/4 of the original pressure?

...........2H2 + O2 ==> 2H2O
I.......30dm^3 + 30dm^3...0
C.........-30...-15.......0
E..........0......15

You had 30 + 30 in a 60 dm^3 container.
After the reaction, and with the same conditions you have 15 dm^3 in the 60 dm^3 container. So the pressure should be just 1/4 (that's 15/60) of the initial pressure.