Two forces act at an angle 120°. the greater is represented by 80kg and the resultant is at right angle to lesser. Find the latter

say the small force is in +X direction. 
So take it as +i so another one (larger force) will make 120 from X axis
so it will make 30-degree form y-axis

so there should not be any x component of forces, so 
80sin30 =Small force 
Small force will be 40kg