Six sirens commence tolling together and toll at intervals of 2, 4, 6, 8, 10 and 12 seconds respectively. In 40 minutes how many times do they toll together simultaneously?

Six sirens commence tolling together and toll at intervals of 2, 4, 6, 8, 10 and 12 seconds respectively. In 40 minutes how many times do they toll together simultaneously? Correct Answer 21 times

Given:

Toll at intervals of six sirens are 2, 4, 6, 8, 10, and 12

Formula used:

L.C.M method

Calculation:

L.C.M of 2, 4, 6, 8, 10 and 12 is 120

120 seconds = 2 minutes

In 40 minutes, they would toll 40/2 ​= 20 times

But then the question says they commence tolling together. So, they basically also toll at the beginning. 

Total tolls together = 20 + 1 = 21 times

∴ They toll together simultaneously 21 times in 40 minutes.