Question

Hn 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.

Answer 1

System of equations:
J     =2M
J+M=21 

2M+M=21
3M=21
M=7

Answer 2

Equation:

M + 2M = 21


Explanation:

M = Mary's age.
2M = Jhon's age is double Mary's age (2 times)
M + 2M = The sum of these two needs to be 21.


Solving the equation:

M + 2M = 21
3M = 21
M = 21 / 3
M = 7


Mary's age is: 7
Jhon's age is two times Mary's: 7 + 7 = 14


Proof : 7 + 14 = 21


Hope it helped,

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