Respuesta :

saraalvi00

Answer:   7 years

Step-by-step explanation:

let x be the age of Mary

so John's age will be 2x

the equation becomes

2x+x=21

3x=21

x=21/3

x=7

devishri1977

Answer:

Step-by-step explanation:

Let Mary's age = x years

John's age = 2 * x = 2x

The sum of their ages = 21

x + 2x = 21

3x = 21

Divide both sides by 3

x = 21/3

x = 7

Mary's age = 7 years