Yes, if you think of the commodity price change on a number line... it going up would be (>), the only other 2 possibilities for it would be going down (<) or remaining the same (=).

So if C = the price goes down today, D = the price goes down tomorrow,

and E = the price stays the same today, F = the price stays the same tomorrow.

Then the probability that the prices go down *or* remain the same today *and* tomorrow is : P((C or E) and (D or F))

P((C or E) and (D or F)) = P(NOT(A and B))

= 1 - P(A and B)

= 1 - 0.5

= 0.5