Four Fours/21/Solutions/1

Puzzle: Four Fours: $21$

Using exactly $4$ instances of the number $4$, the task is to write an expression for $21$, using whatever arithmetical operations you consider necessary.

Solution

$21 = 4! - 4 + \dfrac 4 4$

Sources

• 1964: L.H. Clarke: Fun with Figures ... (previous) ... (next): Solutions: $(46)$
• 1980: Angela Dunn: Mathematical Bafflers (revised ed.) ... (previous) ... (next): $1$. Say it with Letters: Algebraic Amusements: Four Fours