burger fun / Four Fours / Solver
Make 91 with four 4s
The classic four fours puzzle: build an expression from exactly four 4s that equals 91. 91 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 4s and + - × ÷ reaches 91 — the solver checked every possible expression.
Solutions with concatenation and decimals
Concatenation and decimals do not help for 91 either — every combination was checked.
Solutions with square roots, factorials, exponents
4 × 4! - sqrt(4) / .4 = 91
1 distinct solutions found in total, after removing trivial rearrangements.
How 91 compares
91 has 1 distinct solution across all three tiers — #1 scarcest of the 95 solvable targets. Next door, 90 has 23 solutions and 92 has 53 solutions. The nearest impossible target is 93, 2 away — proven unreachable, not just unsolved.
Nothing on the board is scarcer than 91 — its closest rival is 89, with just one solution.
Make 90 with four 4s · Make 92 with four 4s
The Number Lab
Same idea, different constraints: four 9s, four 4s, or any four digits you like.
- Four Nines — the daily puzzle
- Four Nines solver — every target, solved
- Four Fours — the 1881 original
- Four Fours solver — including the impossible five
- Make 24 solver — any four digits
- Today's 24 — one new hand a day
Got what you came for?
Stay for something you did not search for.
- Four Nines — the daily sibling: four 9s, one target, new at midnight
- The Reflex Lab — nine ways to measure your hands and memory, none of them polite about it
- hand me something weird — a random pick from the directory. Could be the horse.
Read up on the 1881 original, pick another number in the solver, or find more free math games.