P(a)+P(b)=0.7
P(a∩b)=0.2
we know P(a∪b)=P(a)+P(b)-P(a∩b)=0.7-0.2=0.5
P{(a∩b')∪(a'∩b)}=? // this is actually P(either a or b but not both)
which is 0.7-2*0.2=0.3
there is another simple method too
let x=only a y=only b and z=both a and b
given (x+z)+(y+z)=0.7 , z=0.2
asked x+z , which is 0.7-0.4=0.3