F is the event that either E or F occurs (or possibly both).You might recall from Unit 6.1 (two tutorials back) that the union of two events corresponds to the inclusive "or" in logic. In other words,
F is the event that either E or F occurs (or possibly both).In this case, EF is the event that either the price of your stocks will increase by at least 50%, or they will stay the same. (This is the same as Choice B above.)
(If you would to review this topic, go back two tutorials to Unit 6.1 by pressing the "prev" button on the sidebar twice...)