Question

Three angles of a quadrilateral are in the ratio 3:4:5 and their mean is 80.find all the four angles.
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Answer 1

as the angles are in a ratio 
the angles are  3x  ,4x , 5x
mean=3x+4x+5x/3=12x/3=4x=80  (80 is the mean given)
x=80/4=20
the angles are 3*20=60
4*20=80
5*20=100
forth angle is 360-(60+80+100)=360-240=120