Take the case of a triangle, you know that the sum of all the interior angles in a triangle is 180 degrees. Say, one of the angles is given to be 40 degrees. You're also given that the other two interior angles have the same measure. How will you find the other interior angles in this case? The sum of all the interior angles is (n-2) * 180, where n is the number of sides of the object (triangle in this case), which is (3-2) * 180 = 180.
If you subtract the known angle 40 from the total of 180, you have 140 degrees for 2 unknown angles. So, each angle is 70 in this case.
Use the same idea for your question that involves a heptagon. The sum of all the interior angles in a heptagon is (n-2) * 180, which is (7-2) * 180 = 900 degrees. Out of these, four angles have a total of 135 degrees per the given problem. Remove 135 from 900, which leaves you with 765. 765 degrees belongs to the 3 unknown angles. So, each angle should be 765/3 = 255 degrees
Hint: If the problem size is large, try to downsize it and form an approach for the smaller problem. You can then extend the idea to the larger problem, like how we did here. We used a triangle (smaller problem) to devise an approach, then used that approach to solve for the heptagon (larger problem)
Hope this helps!
