Answer:
A. 35°
Explanation:
Step 1: Derive an Equation
To answer this question, we will use what's called the Triangle Sum Theorem, which states that the sum of all of the interior angles of a triangle is 180°.
By this theorem, in ΔSRT (the triangle, not the angle), the sum of m∠TSR, m∠RTS, and m∠SRT is 180°. Putting this into a mathematical equation gives us the following:
m∠TSR + m∠RTS + m∠SRT = 180°
Now, from the picture, we can see that m∠TSR = 20° and m∠RTS = 125°. Substituting these values into our original equation gives us:
20° + 125° + m∠SRT = 180°
Step 2: Solve the Equation
Solving for m∠SRT, we get:
20° + 125° + m∠SRT = 180°
145° + m∠SRT = 180° (Simplify the LHS by combining like terms)
m∠SRT = 35° (Subtract 45° from both sides of the equation to isolate m∠SRT)