I have this one question I still dont get what 12/4 plus 12/5 is

Compare the cost of the bike trips from the two companies shown.for both the cost in a linear function of the number of days which statements is true

Express the following as a function of a single angle. cos(60) cos(-20) - sin(60) sin(-20)

Ascientist wants to use a model to present the results of his detailed scientific investigation. why would a model be useful? a )because the model makes the concepts easier to understand b) because the model is easy to put together and to use c) because the model prevents other scientists from asking questions d) because the model requires the audience to pay full attention to it i don't know what to choose because between a and d a is much more likely than d because it only wants to get the attention of the audience. i'm confuse.