Number of independent mechanisms = Number of possible plastic hinges - Number of redundancies
= 2 - 1 = 1
The mechansim is shown in figure.
Δ=5θ=3ϕ
From principal of virtual work,
W.Δ=MPθ+MP(θ+ϕ)
⇒W(5θ)=MPθ+MP(θ+ϕ)
⇒W(5θ)=MPθ+MP(θ+5θ3)
⇒MP=1511W=1.364W