Question

Will medal

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 = 0M - 2J = 0J + M = 21J = M + 2

Answer 1

If John is twice as old as Mary, then J=2M. So you want John's age + Mary's age=21, or J+M=21.

Answer 2

So since you can substitute 2M for J, which equation would it be?