(a) When the three resistors are connected in series:The resultant resistance, Req​=R+R+R=90Ω (b) When the three resistors are connected in parallel:The resultant resistance of the combination,​Req​1​=R1​+R1​+R1​=R3​=303​=101​⇒Req​=10Ω​ (c) When two of the resistors are connected in parallel and this combination is connected in series with the third resistor:Let R' be the resultant resistance of the two resistors connected in parallel to each other. Therefore,​R′1​=R1​+R1​=R2​=302​=151​⇒R′=15Ω​ Now, the net resistance of the combination of the resistors,Req​=R′+R=15+30=45Ω (d) When two of the resistors are connected in series and the combination is connected to the third resistor in parallel:Let R' be the resultant resistance of the series in combination. Therefore,R′=R+R=30+30=60Ω Now, let the net resultant of the combination be Req​. So,​Req​1​=Rn1​+R1​=601​+301​=603​=201​⇒Req​=20Ω​