Find R_{AB}. From A, a 5 Ω resistor leads to node N. Between N and B are a 15 Ω resistor and a 60 Ω resistor (both directly). Between N and node M are 6 Ω and 12 Ω in parallel; between M and B are 20 Ω and 80 Ω in parallel. (answer in Ω)