Question

Three clocks ring simultaneously at 5 p.m. They ring at regular intervals of 12 min, 15 min and 20 min respectively. The time all three clocks ring together next is:

• A. 6 p.m.
• B. 9 p.m.
• C. 5 p.m.
• D. 4 p.m.

Answer 1

The correct option is A 6 p.m.

Time interval after which all thre clocks ring together is the L.C.M. of 12, 15 and 20.

$12=2\times 2\times 3$
$15=3\times 5$
$20=2\times 2\times 5$

L.C.M. is $2\times 2\times 3\times 5=60$

So, the three clocks ring together after every 60 minutes.
After 5 p.m., next the clocks will ring together at 6 p.m.

So option 'a' is correct.