One of the angles formed at the intersection of two parallel secant lines is 40 °

As you know, when two parallel straight lines a and b intersect c, 8 angles are formed. The assignment states that the degree measure of one angle is 40 °. Let’s say ∠1 = 40 °. However, there is no accompanying requirement for the remaining corners. Apparently, it is necessary to determine the degree measures of the remaining angles.
Using the theorem on the equality of the lying angles, we obtain ∠4 = ∠1 = 40 °.
If two parallel lines are intersected by a secant, then the corresponding angles are equal. Therefore, ∠5 = ∠1 = 40 ° and ∠8 = ∠4 = 40 °.
Using the theorem on the sum of one-sided angles, we get: ∠1 + ∠2 = 180 °, whence ∠2 = 180 ° – ∠1 = 180 ° – 40 ° = 140 °. Then we find successively: ∠3 = 140 °, ∠6 = 140 ° and ∠7 = 140 °.