Note that before trying to understand the T-type attenuator circuit it may be helpful to read Efficient Power Transfer (links to http://www.mw0llo.com/efficient-power-transfer.aspx).
In the circuit diagram below the components R2, R3 and R4 form what is known as a "T-type attenuator" inserted between a given sinusoidal voltage generating source of internal resistance R1 and a load resistance R5.

The same circuit diagram is repeated below with some additional labels to assist with the calculations:

For R4 and R5 in series the combined resistance R4,5 is given by:
For R3 in parallel with the series combination of R4 and R5 the combined resistance R3,4,5 is given by:
Simplified, the above gives: