The missing weight in Number Balance is always a single digit between 1 and 9, never zero and never greater than nine. That short, fixed range is not a soft guideline; it is the design contract that defines every round of the game. Each of the five scales presents known number blocks on the left pan, known blocks plus a question-mark block on the right pan, and asks you to press a digit from 1 through 9 that makes both sides equal. Zero cannot balance any of the five equations, and a multi-digit answer like 10 or 14 has no button to enter. Pressing an unavailable key is silently ignored, which keeps the missing-weight slot locked to a positive one-digit answer. The same logic explains why the answer 4 can appear in several rounds and why the game never invites you to type a two-digit weight.
Behind that constraint is a transparent piece of logic. The page stores each round as a small fixture of left weights, right weights, and a single disclosed answer between 1 and 9. A helper routine called getBalanceTotals independently sums the known blocks on the left, the known blocks on the right, and a candidate digit, then a separate routine compares that candidate with the fixture's disclosed answer. If the two sides become equal, the round advances; if they do not, the mistake counter increments and the same scale stays on screen. None of that logic ever returns a value below 1 or above 9, so the question-mark slot never resolves to 0 or to a number such as 12. The rules you see on screen are the same rules the validator runs on every key press, which is why the missing-weight slot behaves the same way in every round.
The Five Fixed Equations Behind the Scales
Number Balance does not generate new puzzles on the fly. It loads five pre-built scales whose left sums, right sums, and correct missing weights are stored in advance, and you can verify each one yourself by adding the visible blocks on each pan. The full set of rounds, with the left total, the right total after the answer is added, and the missing weight that closes the gap, looks like this.
| Round | Left Pan | Right Pan | Left Total | Missing Weight | Right Total After Answer |
|---|---|---|---|---|---|
| 1 | 2 + 3 | 1 + ? | 5 | 4 | 1 + 4 = 5 |
| 2 | 7 | 2 + 2 + ? | 7 | 3 | 2 + 2 + 3 = 7 |
| 3 | 1 + 4 + 2 | 3 + ? | 7 | 4 | 3 + 4 = 7 |
| 4 | 9 + 1 | 6 + ? | 10 | 4 | 6 + 4 = 10 |
| 5 | 3 + 3 + 3 | 1 + ? | 9 | 8 | 1 + 8 = 9 |
Notice that the answer 4 appears in rounds 1, 3, and 4 even though the equations look different. Each scale is independent, so the same digit can balance more than one fixture with no rule against repeating digits. Round 2 produces the smallest valid weight in this set, which is 3, and round 5 produces the largest valid weight, which is 8. No round asks for 0, no round asks for 10, and no round invites a two-digit entry. The whole table is a closed set: every possible missing weight for this fixed board is already listed.
How to Solve a Number Balance Round Step by Step
- Add every visible block on the left pan in your head or on paper and remember that total.
- Add the visible blocks on the right pan the same way, ignoring the question-mark block for now.
- Subtract the right total from the left total to find the exact amount the missing weight must contribute, or add up to the left total if you find that simpler.
- Press the matching digit from 1 through 9 on your keyboard, or click the same button on screen.
- Check that the new right total equals the left total; if it does, the round advances and you earn 200 points.
The clean keyboard route through all five rounds is 4, 3, 4, 4, 8, so a player who trusts the math can clear the board in five correct presses and finish at 1,000 points. Worked example for round 1: the left pan shows 2 + 3, which equals 5; the right pan shows 1 plus a question mark, so the missing weight must be 4 because 1 + 4 = 5. The same pattern works on every other scale: sum the known blocks, compare, and press the single digit that closes the gap. If the missing weight would have to be 0 or larger than 9 to balance a given round, that round would simply not exist in the fixed set.
What Happens When You Press a Wrong or Out-of-Range Key
Because the accepted range is hard-coded to 1 through 9, the game treats every other input the same way: it does nothing visible. Pressing 0 leaves the state unchanged because no fixture accepts 0, and pressing a multi-digit number is impossible since the keyboard can only register the keys that exist on screen. The number keys 1 to 9 are the only inputs the page listens for during a live round, and the same nine buttons are mirrored as clickable elements for mouse and touch users. If you press any key outside that set, the current scale stays put, the score stays put, and the mistake counter does not move. The two-strike rule only counts wrong digits inside the 1-to-9 range, not out-of-range keys.
A wrong digit inside the accepted range behaves differently. The same scale stays visible, the mistake counter becomes 1, and you can recalculate both sides before pressing again. A second wrong answer inside the same range drives the mistake counter to 2 and creates a terminal deadlock: further digits are ignored until you press Restart. That gives you one recoverable mistake per run and a clear signal when the run has ended. There is no hidden timer and no speed penalty, so the failure path is observable and fair. You always know whether the next press will count, fail silently, or freeze the board.
Scoring, Round Progression, and Restart
Each correct scale awards 200 points, so five correct rounds produce the maximum 1,000. A wrong digit costs no points on its own, but a second wrong digit ends the run before the remaining 200-point increments can be collected. The score and mistake counter reset together: Restart returns to the first 2 + 3 scale with zero points and zero mistakes, which is useful when you want to re-run the same fixed board to study where you slipped. Your best completed score is stored only in the current browser's localStorage, so it stays private to your machine and disappears when you clear site data. Double-pressing Escape reveals a simulated spreadsheet overlay that doubles as a boss key, and a second double-Escape returns you to the unchanged scale without losing progress. None of these features depend on a random generator, an account, an audio cue, or a download, so the score you see is the score your logic earned.
Other Browser Logic Puzzles Built on Single-Digit Logic
If the single-digit constraint appeals to you, several other Lizely puzzles use a similar minimal-digit vocabulary. Number Sequence Quiz walks you through five short arithmetic patterns with the same 1-through-9 token set and a two-mistake limit, which makes it a natural follow-up once you have finished the Number Balance board. Addition Maze asks you to pick a route through a compact number grid whose visited cells must add to an exact target, which is a useful companion if you want to practice summing small digits under a fixed rule. Both keep the keyboard front and center, just like Number Balance, and neither requires a download, an account, or a network call once the page is open.
For a direct return to the missing-weight puzzle, the full Number Balance board lives at Number Balance, where the same five fixed scales, the same 1-to-9 keypad, and the same 1,000-point ceiling are ready to play.