Hi Aspirant!

As we know,

Vapour pressure of solution{P(total)} = P(A) + P(B), X(A)P(A) + X(B)P(B)  {=>P(A)=X(A)p(A)}

The vapour pressure of component 1 , = P1=Y1P(total)

Here, y is the mole fraction of component 1 in vapur phase.

As, Vapour pressure of A (pure liquid) = P(A)= 100mm

and ,  Vapour pressure of B (pure liquid) = P(B)= 160mm

Therefore, the total vapour pressure of solution = P(A) + P(B)

P(Total) = X(A)P(A) + X(B)P(B) = (2/5)*100 + (3/5)*160 = 136mm

Also, P(A) = Y(A)P(total)    {Y(A) is the mole fraction of A.}

Mole fraction of A = Y(A) = (P(A)/P(total)) = 40/136 = 0.294

Therefore, Y(B) = 1 - Y(A) = 1 - 0.294 = 0.706

Thankyou!