Imagine you have a 10 V power source and two resistors, one 20 Ohm and the other 5 Ohm. How would you arrange them to generate an average current of 2.5 A? How much current goes through each resistor?

There are only two ways to arrange the resistors:

series: R=20+5 = 25Ω, making a current of 10/25 = 0.4 A
parallel: 1/R + 1/5 + 1/20 = 1/4, so R = 4Ω, making a current of 10/4 = 2.5 A
So now you know that each resistor has a 10V drop across it, and you can easily figure the current in each.