The perimeter of a triangle is the sum of all side lengths of the triangle. The numerical expression for the perimeter of Stephanie's triangle is: 
Let the sides of Juan's triangle be x, y and z. So:

The perimeter (J) of Juan's triangle is calculated by adding all sides.
So:

This gives:


From the question, we understand that:
The perimeter (S) of Stephanie's triangle is half that of Juan.
This means that:

Substitute 25 for J

Hence, the numerical expression for the perimeter of Stephanie's triangle is: 
Read more about perimeters at:
brainly.com/question/11957651
Answer:
The missing reason is Subtraction Property of Equality.
Step-by-step explanation:
⇒ (Given)
On Solving we get;
Multiplying 2 on both side we get;

⇒ (Multiplication Property of Equality)
Now Subtracting Both side by 10 we get;

⇒ (Subtraction Property of Equality)
Now Dividing both side by 3 we get;

⇒ (Division Property of Equality)
Hence the missing reason is Subtraction Property of Equality.