How many liters of 15% acid and 33% acid should be mixed to make 40 liters of 21% acid solution?

Need on this one asap im almost done with this subject

Divide. reduce the answer to lowest terms.5 2/3 ÷ 3 1/9

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.