I think that addition falls into the category of "Things that you should have learned in school, had you been paying attention", but for whatever reason, you are here.
Step 1: The basics
that is to say that 50+25=75, and 25+50=75
And perhaps more importantly, addition is associative, meaning that you can add more than two numbers together and get the same result.
Which is to say that 25+25+25=75, and 15+15+15+15+15=75
But how does it happen? How do you add? I will try and explain it in the next few steps