Factors and multiples

Question

Four traffic lights along a street turn red at regular intervals of 1 minute,1 minute 10 seconds,1 minute 18 seconds and 1 minute 31 seconds respectively.Occasionally all four traffic lights will turn red simultaneously.If all traffic lights turned red simultaneously at 0830,find the next time it occurs again.
1 min=60 secs
1 min 10 secs=70 secs
1 min 18 secs=78 secs
1 min 31 secs=91 secs

Answer 1

You are just asking the least common multiple of the set $$ \{60,70,78,91\}\,\,\text{(seconds.)} $$