Question

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 represents the problem.
J - 2M = 0
M - 2J = 0
J + M = 21
J = M + 2

Answer 1

ANSWER

The system of equations are,

$J - 2M = 0$
and

$J + M = 21$

EXPLANATION

Let J = John's age and M = Mary's age

The first statement says,

"John is twice as old as Mary."

We can write this mathematically as,

$J = 2M$

or

$J - 2M = 0...eqn1$

The second statement says,

"the sum of their ages is 21." Mathematically, we can write this one too as,

$J + M = 21...eqn2$

Answer 2

J = 2m
J + M = 21
J + M = 21 is your answer.