\(U _{1}+ U _{2}= U _{\text {mixutre }}\)
\(\frac{ f }{2} P _{1} V _{1}+\frac{ f }{2} P _{2} V _{2}=\frac{ f }{2} P \left( V _{1}+ V _{2}\right)\)
\(\frac{ f }{2}(2)(4.5)+\frac{ f }{2}(3)(5.5)=\frac{ f }{2} P (4.5+5.5)\)
\(\Rightarrow P =2.55= x \times 10^{-1} atmp\)
So \(x =25.5 \approx 26\) (Nearest integer)