Even upside down, a bat can continue to use its digital watch, because some hours are the same even when viewed in reverse. For example, this is what happens with 12:21.
Let's call these hours reversible.

What is the shortest interval between two reversible hours?

Source:Rudolpho Korchan.

(In reply to Colon included by Jer)

No, Sir.

None of those is the shortest interval.

Posted by Ady TZIDON on 2015-12-08 13:37:55