\left.\begin{array}{l}{[\text { Arithmetic. Mental calculation, etc. }]} \\ {[\text { Arithmetic progression }}\end{array}\right]

When little Clive approached his grandfather's cuckoo clock, it was 12:05.

Clive started turning the minute hand until the hour hand returned to its original position. How many "cuckoos" did his grandfather count during this time from the next room?
Since the clock strikes every hour, Grandpa counted 1+2+3+...+12 = 78 "cuckoo".

## Answer

Grandpa counted 78 "cuckoo".