Liquids A and B form an ideal solution. A certain solution contains 25 mole % of A, whereas the vapor in equilibrium with the solution at 25 degrees C contains 50. mole % of A. The heat of vaporization of A is 5 kcal/mol; that of B is 7 kcal/mol.

Question

a) Calculate the ratio of the vapor pressure of pure A to that of pure B at 25 degrees C.
b) Calc. value for the same ratio at 100.degrees C.

My first thought was this:
P1(a)=X1(a)P1*(a)
P2(a)=X2(a)P2*(a)
P1(b)=X1(b)P1*(b)
P2(b)=X2(b)P2*(b)

where P1(a)=P2(a)=P1(b)=P2(b)
but if I did
.50P2*a=.25P1*a,
P1/2*a would = 0, as would P2/1*b if I did the same for b.

So how do I really approach this?