Question

$5$ bells start tolling together and toll at intervals of $2,4,6,8$ and $10$ seconds, respectively. How many times do the five bells toll together in $20$ minutes?

• A. $10$
• B. $11$
• C. $12$
• D. $15$

Answer 1

The correct option is B $11$

The times after which the bells ring together must be equal to the LCM of $2,4,6,8,10$ and the LCM of these numbers is:

$2\times 2\times 2\times 3\times 5=120$ seconds

We know that there are $60$ seconds in one minute, therefore, $120$ seconds will be same as $2$ minutes.

Now, we find the number of times that the bells toll together in $20$ minutes as follows:

$\frac{20}{2}+1=10+1=11$

(Note: $1$ is added because the bells are tolling together)

Hence, the bells toll together $11$ times in $20$ minutes.