Three Fridays

Three Fridays of a month fall on even days, such as the 8th or the 24th. On which day of the week does the 21st of this month fall?
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Three Fridays of a month fall on even days, such as the 8th or the 24th. On which day of the week does the 21st of this month fall?
How many 10-digit numbers with different digits are there?
Prove that a certain number can never be prime.
Show that five lattice points can always be connected in a specific way.
A good example from divisibility and prime number theory.
A slight variation on an old logic puzzle about proverbial liars and truth-tellers.
Is it possible to find five positive whole numbers 𝑎, 𝑏, 𝑐, 𝑑, 𝑒 such that there is no subset of them with a sum divisible by 5?