List of top General Aptitude Questions on Number Systems

Three bells P, Q, and R are rung periodically in a school. P is rung every 20 minutes; Q is rung every 30 minutes and R is rung every 50 minutes.
If all the three bells are rung at 12:00 PM, when will the three bells ring together again the next time?

p and q are positive integers and \[ \frac{p}{q} + \frac{q}{p} = 3, \] then, \[ \frac{p^2}{q^2} + \frac{q^2}{p^2} = \]