coucou j’ai un exo de maths, est ce que qqn peut m’aider svp ? Jade's watch rings every 12 hours and her alarm rings every 15 hours. They both rang on September

coucou j’ai un exo de maths, est ce que qqn peut m’aider svp ?

Jade's watch rings every 12 hours and her alarm rings every 15 hours. They both rang on September 21st at 6.30 pm.

• When will they ring together again?

merci d’avance !!