When an angle is trisected, it is divided into 3 equal parts. That is,
... 3x -4y = x - y . . . . . ∠NOP = ∠POR
... x - y = y - 10 . . . . . ∠POR = ∠ROS
Add y to the second of these equations and you get
... x = 2y - 10
Substitute this into the first equation and you get
... 3(2y -10) -4y = (2y -10) -y . . . . substitute 2y-10 for x
... 2y -30 = y -10 . . . . . . . . . . . . . simplify
... y = 20 . . . . . . . . . . . . . . . . . . . add 30-y
... y - 10 = 20 -10 = 10 . . . . . . . . .substitute the value of y into the expression for the measure of angle ROS
m∠ROS = 10
