bond help please

[senthi]

bond more 4th maths,paper 7, Q:18

three roadside flashing beacons are all switched on at the same time.one flashes every 6 seconds,one every 9 seconds and the third every 12 seconds. how many seconds after being switched on do they flash at the same time?

the answer is 36 seconds.

can any one explain the work out please?thankyou

[hermanmunster]

easiest way to look at these is by writing down the multiples of the longest time and seeing if the others will have been flashing at the same time:

12 6 yes 9 no
24 6 yes 9 no
36 6 yes 9 yes

[senthi]

thank you very much