Question

18 The first bell rings every 25 minutes and the second bell rings every half an hour. The first bell rings at 5.50 a.m. and the second bell rings at 6 a.m. (a) When will they next ring together? (b) How many times will they ring together before 6 p.m.?​

Answer 1

So first you find the LCM, or least common multiple, (can be called LCD, least common denominator).

The LCM of 20, 30, and 50 is basically

10⋅2⋅3⋅5

because you factor out the 10 since that is a common factor.

10⋅2⋅3⋅5=300

This is the number of minutes. To find the number of hours, you simply divide by 60 and get 5 hours. Then you count 5 more hours from 12:00 pm and get 5:00 pm.