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.
a)J - 2M = 0
b)M - 2J = 0
c)J + M = 21
d)J = M + 2

There can be more than one answer :)

Answer 1

So first off, is there an equation that expresses that the sum of the ages is 21?

Answer 2

J = 2M=>
=>J - 2M = 0
J + 2M = 21 (sum of both is 21)
So:
J - 2M = 0
J+2m = 21