Alarm clock

The old watchmaker has a unique digital alarm in its collection that rings whenever the sum of digits of the alarm is equal to 21. Find out when the alarm clock will ring. What is their number? List all options . ..

Correct answer:

n =  16

Step-by-step explanation:

n1=07:59 n2=08:49 n3=08:58 n4=09:39 n5=09:48 n6=09:57 n7=16:59 n8=17:49 n9=17:58 n10=18:39 n11=18:48 n12=18:57 n13=19:29 n14=19:38 n15=19:47 n16=19:56 n=16

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