Why do we use letters in mathematics?

We use letters to substitute for unknown numbers. I hope this helps. :)

I would say we use letters in mathematics so you can understand the problem more or whats really happening for example if they ask you question about how many people went to the party and bought stuff without the letters and only numbers you'll be confused...like umm...how many people went what are the stuff they bought.Or another example is if the question only had an empty graph.People would be like what do I have to do with the empty graph.What's the problem etc.So math problems without letters would be confusing.

B, 0.4 repeats so it is rational and square root of 16 is 4 so that is rational as well.