I think it is the 3rd one but I might be wrong...
90 degrees, a right triangle has 1 right angle which is 90 degrees
The tangent line is perpendicular to the radius from the center of the circle to the point of tangency.
ΔDCE is similar to ΔDBA
DC/DB=DE/DA
15/50=DE/40
DE=12
Let:
100%---------------->24h
30%------------------>xh
Using cross multiplication: