The options for above question is

a.1 hrs

b.2 hrs

c.3 hrs

d.4 hrs

Logically the next time when they ring together will be a factor(least possible minutes) common to all 3 bellls

so we need to take LCM of 9 ,12 and 15

Calculating from the above steps

LCM = 2 × 2 × 3 × 3 × 1 ×1 × 5 = 180

∴ Time after which the three bells will ring together next = 180 minutes

using formula 60 minutes = 1 hour

∴ 180 minutes = 180 / 60 = 3 hours

the ans is c. 3 hrs