Two clocks begin to strike 11 together. The first clock strikes every stroke in 3 seconds while the second strikes in 2 seconds. What is the interval between the first clock’s fifth stroke and the second clock’s seventh stoke?
(A) 1 sec
(B) 2 sec
(C) 5 sec
(D) No time interval
It seems the answer is straight forward by $1=3*5-2*7$, i.e. $(A)$. However, the answer turns out to be $(D)$. Why is that?
Because the first stroke is the one at 11 o'clock. They will strike together the first time. So you get one less time interval each and thus you have $0 = 3\cdot 4 - 2\cdot 6$.