Question

Norrie sees two lights flash at the same time, then one of them flashes every 4th second, and the other flashes every 5th second. How many times do they flash together during a whole minute?

Answer 1

Let us assume Norrie sees two lights flash at the same time at 'time'=0 sec.

Frequency of first flash (I) = Every 4th second.

Frequency of second flash (II) = Every 5th second.

Let us assume the two flashes will simultaneously occur after 't' seconds.

To calculate the time 't' we need to take the LCM of 4 and 5.

( LCM is Least common multiple and it is defined as the smallest number that is a multiple of both 4 and 5 here )

LCM of 4 and 5 = 20

( As you can see 20 is the smallest number that is a multiple of 4 and 5).

so the flashes will occur simultaneously after every 20 seconds.

Now according to the problem posed by the question we need to find how many times flashes will occur together in 1 minute.

1 minute = 60 seconds

so number of flashes that occur together within 1 minutes will be = = 3 Times.

So the final answer is both the flashes will occur together 3 Times in whole 1 minute.