Answer:
So here we have 3 adjacent angles, and add them all up they would equal a right angle as shown in the diagram.
POT is the right angle which can be seen by the square
POQ is the first angle
QOS is the second
SOT is the third
Now solve it.
We know that it has to be equal to 90 since thats the final reasoning.
4a + 20 + 3a - 10 + a = 90
Solve.
So you’d get 10.
If you want to elaborate then just say all adjacent angles has to add put o 90 since