# The ages of two friends is in the ratio 5:6. After how many years will the ages be in the ratio 7:8? a)20year b)12year c)15year d)10year

So $\displaystyle\frac{{5x + k}}{{6x + k}} = \frac{7}{8}$
$\Rightarrow 40x + 8k = 42x + 7k$
$\Rightarrow$ k = 2x