Mohan has an antique clock which strikes and makes loud gong sound every hour. It strikes the exact number of times indicating the time of the day or night. His clock takes seven seconds to strike Seven O' clock, how many seconds will his clock take to strike Eleven O'clock?

After making the first gong, it has a gap of 6 time periods to complete the 7 gongs. 

In other words, let us assume that at t=0, the first loud gong took place at 7 O' clock. After a period of 6 equal time periods, all the 7 gongs were made.

So, 6 time period= 7 seconds.

1 Time period= $$\ \frac{\ 7}{6}\sec$$

Similarly, at 11 O' clock, after the first gong, the remaining 10 gongs will sound at an interval of 10 equal time periods. 

So, time taken= $$10\times\frac{\ 7}{6}\sec$$= $$\frac{\ 70}{6}\sec=\ 11.67\ $$ seconds.