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Let Mary's age will be x years.
Then the grandmother's age is  (9x - 11) years.
According to the condition,
    x + (9x-11) = 69.
Simplify and solve for x
    10x = 69 + 11
    10x = 80
      x = 80/10 = 8.
ANSWER.  Mary is 8 years old;  her grandmother is  69-8 = 61 years old.
CHECK.   8*9 - 11 = 72 - 11 = 61 years.  ! Correct !
Solved.