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
J = M + 2
M - 2J = 0
J + M = 21
If their ages combined equals 21 and john is twice Mary’s age then johns age takes up 2/3 of 21. that would b 14. 14 is 2 x 7. and Mary is 7. 14+7=21. Mary is 7 years old
