Four Fours/90/Solutions/2

Puzzle: Four Fours: $90$

Using exactly $4$ instances of the number $4$, the task is to write an expression for $90$, using whatever arithmetical operations you consider necessary.

Solution

$90 = \dfrac {4 \times 4} {.4 \times {.\dot 4} }$

Sources

• 1980: Angela Dunn: Mathematical Bafflers (revised ed.) ... (previous) ... (next): $1$. Say it with Letters: Algebraic Amusements: Four Fours