You must be very new to electronics to ask such a question, so i'll try to explain it as simple as possible.
A voltage (or potential) drop is always calculated with respect to a start and end point. The circuit is a loop, so the start and end points are the same, hence net voltage drop around the loop is also zero.
If the battery gives voltage rise of 20V, and R3 give voltage drop of 4.5V, then R1 & R2 combined should give a voltage drop of 15.5V. The rest is ohm's law.
This is a simple circuit. There are times whereby the circuit is more complicated, and you would have to use variables to replace unknowns and solve via simultaneous equations.
Here's a good link for multiple loop systems.
**broken link removed**