Because national heroes are in abundance
So he knows there are definitely 66 trouts as he caught and tagged them.
The next day he caught 52, but 13 of them were already tagged, so were part of the number of trout he caught before.
So he can expect that:
66+(52-13)= 105 trout are in the lake
Let the measure of the unknown angle is x °
Supplementary angles sum up to 180°. So the supplementary angle of x° will be (180 - x)°
According to the given data, measure of x is 40° less than 3 times the measure of its supplementary angle (180 - x). So we can set up the equation for this scenario as:

So the degree measure of the required angle is 125°.
The answer is 51°.
The top two lines are parallel to each other. Therefore alternate interior angles theorem applies. The bottom right corner that is underneath the line (for lack of a better description) is also 58°.
We also know that a triangle equals 180°. So 58+71= 129°.
So to find that missing angle we subtract 129 from 180 to get the 51°.