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**Hint:**In this type of questions which are related to age of person, the first thing we do is to assume variables for the present age of the persons given in the question. And then for some years we have to increase them by that particular number of years and for some years we have to decrease them by that particular number of years.

**Complete step by step solution:**Suppose the present age of Mary is \[x\] years.

Therefore after $15$ years, the age of Mary will be = $x + 15$ years.

According to the given condition in question-

Mary’s age after $15$ years =$4 \times $ (Mary’s present age)

$ \Rightarrow x + 15 = 4 \times x$ [By our supposition]

$ \Rightarrow x + 15 = 4x$

$ \Rightarrow 15 = 4x - x$

$ \Rightarrow 15 = 3x$

$ \Rightarrow \dfrac{{15}}{3} = x$

$ \Rightarrow x = 5$

Hence the present age of Mary is $5$years.

**Note:**There are more chances to make a mistake in the solution if you do not suppose the present age to be a variable and you do assume the variable for future age or past age of the given persons.

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