Mary's age is twice her brother's age and half her sister's age. How old is Mary, in years, if the sum of the three ages is 28 years?
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Let Mary's age = M
Her brother's age = M/2
Her sister's age = 2M
M + M/2 + 2M = 28 simplify
3.5M = 28 divide both sides by 3.5
M = 8 ( Mary is eight years old )