A person's present age is two-fifth of the age of his mother. After 8 years, he will be one-half of the age of his mother. How old is the mother at present ?
Answer: C
Let the mother's present age be \(x\) years.
Then, the person's present age = \(\frac{2}{5}x\)
\(\frac{2x}{5+8}x=\frac{1}{2}\left ( x+8 \right )\)
\(2(2x+40)=5\left ( x+8 \right )\)
\(x=40\)