15. Three mark example problems III

There are four mobile phones in a house. At $5$. a.m, all the four mobile phones will ring together. Thereafter, the first one rings every $15$ minutes, the second one rings every $20$ minutes, the third one rings every $25$ minutes and the fourth one rings every $30$ minutes. At what time, will the four mobile phones ring together again?

 

 

The four mobile phones will ring together at 10:00 a.m5:00 a.m5:00 p.m10:00 p.m.