For this problem we recall that:
[tex]P(A\cup B)=P(A)+P(B)-P(A\cap B)[/tex]Therefore,
[tex]P(A\cap B)=P(A)+P(B)-P(A\cup B)[/tex]and we recall that A and B are independent if and only if
[tex]P(A\cap B)=P(A)\cdot P(B)[/tex]We know that P(A)*P(B)=0.7*0.2=0.14. For the variables to be independent: The probability of buying bread and cheese is 0.14.