Need help in math ?????

Question

a light flashes every 1 minute 15 seconds. another flashes every 1 minute 40 seconds. suppose they flash together at certain time. what is the shortest amount of time that will elapse before both lights will again flash together?

Answer 1

1 min 15 s = 75 s

1 min 40 s = 100 s

Lowest Common Multiple of 75 and 100

= 3 x 25 x 4 = 300 s = 5 min