newbe

Dear friend,
I am getting confused in this question;what i learned till now is that number of proton+number of neutron=atomic mass. But while searching the net i got this equation
atomic mass = mass a x fract a + mass b x fract b. can any one tell me which is the correct (not approximate) method for finding atomic weight.

and my second question

is weight of proton+weight of neutron= atomic mass is this equation correct

advance thanks for your help