sin(80-θ)/sinθ = 2.sin20.sin40/sin10 = cos60-cos20 / sin10
= cos10-2cos20.cos10 / 2.sin10.cos10
= cos10-(cos30+cos10) / sin20
=> sin(80-θ)/sinθ= cos30/sin20 = sin60/sin20 => θ = 20. (here -ve sign has to be neglected since from the figure it is clear that θ is acute angle)
hope it's clear now :)