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

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

Answer 2

ANSWER

The system of equations are,


`J - 2M = 0`
and


`J + M = 21`

Step-by-step 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`