Make 66 with four 9s

The classic four nines puzzle: build an expression from exactly four 9s that equals 66. 66 cannot be made with + - × ÷, concatenation, or decimals (the solver checked every expression) — solutions need square roots, factorials, or exponents.

Solutions using only + - × ÷

No expression made of four plain 9s and + - × ÷ reaches 66 — the solver checked every possible expression.

Solutions with concatenation and decimals

Concatenation and decimals do not help for 66 either — every combination was checked.

Solutions with square roots, factorials, exponents

99 × sqrt(9)! / 9 = 66
99 / 9 × sqrt(9)! = 66
9 × 9 - 9 - sqrt(9)! = 66
9 × 9 - sqrt(9)! - 9 = 66
9.9 × sqrt(9)! / .9 = 66

…and 71 more at this tier.

76 distinct solutions found in total, after removing trivial rearrangements.

Make 65 with four 9s · Make 67 with four 9s

The Number Lab

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