Maths

• September 30th 2012, 08:38 AM
Spixol123
Maths
In three years time Ann will one third of her mother's age. In another 18 years from now, Ann will be half her mother's age. How old is her mother now??
• September 30th 2012, 09:14 AM
Soroban
Re: Maths
Hello, Spixol123!

Quote:

In three years time Ann will one-third of her mother's age.
18 years from now, Ann will be half her mother's age.
How old is her mother now?

Let $A$ = Ann's age now.
Let $M$ = Mother's age now.

In three years, they will both be three years older.
. . Ann will be $A + 3$ years old.
. . Mother will be $M + 3$ years old.
At that time, Ann will be one-half her Mother's age.
. . $A + 3 \:=\:\tfrac{1}{2}(M + 3) \quad\Rightarrow\quad M - 3A \:=\:6$ .[1]

In eighteen years, they will both be eighteen years older.
. . Ann will be $A + 18$ years old.
. . Mother will be $M + 18$ years old.
At that time, Ann will be one-third her Mother's age.
. . $A + 18 \:=\:\tfrac{1}{3}(M + 18) \quad\Rightarrow\quad M - 2A \:=\:18$ .[2]

We have: . $\begin{Bmatrix}M - 3A &=& 6 & {\color{blue}[1]} \\ M - 2A &=& 18 & {\color{blue}[2]} \end{Bmatrix}$

Subtract [2] - [1]: . $A \:=\:12$

Substitute into [2]: . $M - 2(12) \:=\:18 \quad\Rightarrow\quad M \:=\:42$

Her mother is 42 years old now.
• September 30th 2012, 09:22 AM
Spixol123
Re: Maths
Wow that's a lot to remember! Thanks for your help!!
• September 30th 2012, 10:09 AM
Spixol123
Re: Maths
why in 3 years old will Ann be half her mothers age??