so there will be a time to "ding" and a time for the lag between "ding"s. So let's call ding time x and lag time y

example given is 22 seconds for 6 dings from begining of first ding to end of last ding. This means we will have 5 lags between each ding.

so 6x+5y = 22 sec

we are told dings and lag times are equal so x=y

6x+5x=22
so x=2

now question asks us for time elapsed at 12
so 12 dings and 11 lags or = 12x + 11x = 23x = 23(2) = 46 seconds

so answer is d

hope this helps

I get the same answer. Did it the following way

|=stroke _ = interval

For 6:00
|_|_|_|_|_| = 11 total events in 22 seconds.. 1 event = 2 secs

For 12:00, there will be 23 events
So 23 x 2 = 46 s

Thanks guys. Very helpful. 46 is the right answer.