Consider the experiment of tossing 2 coins. Let A be the event of obtaining 2 heads and B, the event of obtaining at least one tail. Determine $A\cup B.$

Recall that the sample space S for tossing 2 coins is S = {HH, HT, TH, TT}. Based on the given conditions, A = {HH} and B = {HT, TH, TT}. Thus, $A\cup B$ = {HH, HT, TH, TT}, which is equal to the sample space S.