Question

this is a hard one and you can select more than one answer

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
and
J + M = 21

Answer 2

how did you get it

Answer 3

well if John is twice as old as Mary...then it makes sense that John's age, J, is twice as much as Mary's age, M, right? J = 2M
then if the sum (added together) of John's and Mary's ages is 21, then J + M = 21
:)