Three bells ring respectively at an interval of $15$ seconds, $20$ seconds and $24$ seconds. If they ring continuously for$12$ minutes then how many times during this period, will they ring together? 

Which of the following fractions is the greatest of all?
$\frac{7}{8} ,\frac{6}{7} ,\frac{4}{5} ,\frac{5}{6}$.

The traffic lights at three different road crossings change after every $48$ seconds, $72$ seconds and $108$ seconds respectively. If they all change simultaneously at $8:20:00$ hours, then they will again change simultaneously at: