for an ideal gas/liquid goes Raoults Law:
partial pressure of the gas = gas pressure of the pure liquid compound * mole fraction in the solution
pi = Pi * Xi
this will give you the partial pressures of the compounds in your mixture
then you have mole fraction of B in the gas mixture by partial pressure of B / total pressure of the gases pB / ( pB + pA )
Daltons law is incorrectly used by you, because the 200 and 400 torr are not partial pressures, but pressures of the pure compounds.