John is twice as old as Mary. The sum of their ages is 21. How old is Mary? Let J = John's age and M = Mary's age. Select the system equations that represent the problem.

J = 2M

J + M = 21

Step-by-step explanation:

M = Mary's age

J = John's age

"John is twice as old as Mary."

J = 2M

"The sum of their ages is 21."

J + M = 21

The system of equations is:

J = 2M

J + M = 21