Here is the step-by-step solution

 

​3 Koalas = 120 kg

​1 Koala = 120 ÷ 3 = 40 kg

​1 Koala + 1 Kangaroo = 70 kg

​1 Kangaroo = 70 - 40 = 30 kg

​1 Kangaroo + 2 Rhinos = 140 kg

​2 Rhinos = 140 - 30 = 110 kg

​1 Rhino = 110 ÷ 2 = 55 kg

​Answer: The weight of 1 rhino is 55 kg.