I'm really struggling to understand the formulae used to express the capacitance or resistance of series capacitors or parallel resistors. I know it must be really simple but i just cannot understand how the text books get to the final figures!

Say for instance you had 2 capacitors wired in series that were 10 μ F & 20 μ F how do i find the total capacitance for the circuit.

Please help this is driving me insane, i keep just staring at the formulae feeling pretty dumb.

Thanks