Can somebody help me with those Math questions?

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.

What is the missing reason in the proof? segment addition congruent segments theorem transitive property of equality subtraction property of equality?

Find two numbers if their sum is 91 and the ratio is 6: 7?