To do this, you need to use the combined gas law 
(P1)(V1) / T(1) = (P2) (V2) / 2
For temperature, convert to Kelvins
T1= 20.1C + 273 
T1= 293.1 K 
T2= 40 +273 
T2= 313 K 
Now sub the numbers into the formula 
(1) (0.78) / 293.1 = (1.2) (V2) / 313 
0.00266 = 1.2(V2) / 313 
0.833 = 1.2(v2) 
V2= 0.69 L is the new volume