How much interest does a 646$ investment earn at 8% over 7 years

What is the best way to answer questions with a x expression in the question. for example 3x + 10x=?

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.

Jordan got 27 out of 90 correct on his test . what fraction of the marks did her get correct