Here it is
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QuoteA 100 ml vessel containing O₂(g) at 1.0 atm and 400 K is connected toa 300 ml vessel containing NO(g) at
1.5 atm and 400 K by means of a narrow tube of negligible volume where gases react to form NO2. Final pressure of mixture will be-
(A) 1 .125 atm (6) 0.125 atm (C) 1 atm (0) 1.5 atm

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O2
P=1atm
V=100mL
T=400K

NO
P=1.5atm
V=300mL
T=400K


Moles of O2 = PV/RT
                   = 1*0.100/RT
                   =    0.1/RT

Moles of NO = PV/RT
                   = 1.5*0.300/RT
                  =  0.45/RT


Balanced  equation is

                        O2  +2 NO       -----> 2NO

Initial moles   0.1/RT     0.45/RT            0


                   Final  moles   0     0.25/RT                  0.20/RT



Total moles finally =  0.25/RT            +      0.20/RT= 0.45/RT   
Total Volume  = 400mL =0.400L   
        P (total)= n(total)RT/V
Plug in all values in proper units
You will get the right answer.