btw, how old are you?
Thanks but what do you ask my age?
Most likely so we'd know how in-depth the explanation should be. Throwing a bunch of equations at a 13 year old who hasn't take Trig yet might be frustrating, whereas a 20 year old in College might understand it better. So, if we know you're young (or older but not strong in the math department) we can explain the fundamentals first.
You should look into a program like iCircuit to help you visualize things. There's also a good JavaScript based circuit simulator that will run right in your browser.
Here's your initial schematic, simulated:
Now, we can simplify it by replacing the parallel pairs of resistors with their equivalent:
But wait! We can drill down even further by replacing the series pair with a single resistor:
Hopefully that helps you visualize it a bit better.