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Angle 1 + angle 6 is equal to 180° because they are on the same line.
so if angle 6 is 125° so angle 1 is:
180 - 125 = 55° :)))
i hope this is helpful
have a nice day 

⇒ Solutions 

All the interior angles of a triangle will always add up to 180°. Angle 1 plus angle 6 will equal 180°. If angle 6 is equal to 125° to find angle 1 you will need to subtract 125° from 180°. 

180° - 125° = angle 1 
180° - 125° = 55° 

Angle 1 = 55° 

Check your work 

To see if the answer is correct 125° + 55° have to equal 180°
125° + 55° = 180° 

The answer is correct