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The component of force of gravitation for a body of mass m along the inclined surface making an angle Ф with the horizontal is :   Fg =  m g Sin Ф 

The force of static friction Ff just equals Fg as the body just starts to slide down.
      Ff = μ N = μ m g Cos Φ = Fg = mg Sin Φ

Hence, μ = coefficient of static friction = Tan Φ = 2.5 cm / 12 cm = 0.2083