Question

The traffic lights at 4 different crossings at intervals of 48, 72, 90 and 108 seconds respectively. If these lights change together at 7:05 p.m. , when will they change together again ?

Answer 1

Lights will change together after `2160` seconds or `36` minutes i.e. at `7:41` pm.

Explanation:

In such cases we should take the LCM of `48,72,90` and `108`

As `48=2xx2xx2xx2xx3`

`72=2xx2xx2xx3xx3`

`90=2xx3xx3xx5`

and `108=2xx2xx3xx3xx3`

LCM is `2xx2xx2xx2xx3xx3xx3xx5=2160`

Hence lights will change together after `2160` seconds or `36` minutes i.e. at `7:41` pm.