Brain Test 5 Level 65

Overview

This is a Light-Up (Akari) logic puzzle spread across five notebook pages. The page reads "Light up all numbered tiles." and shows a 4×4 grid. Red tiles are forbidden walls that cannot be lit. White tiles carry numbers that tell you how many lit bulbs must sit in the four cells adjacent to them. Two cells are already lit green, showing the starting state. A red X marks a wrong placement. The grid grows larger each round, and you must satisfy every number constraint while ensuring no two bulbs shine on each other. The overall approach is to deduce bulb positions from the numbered clues and light every white tile.

• Difficulty: Hard
• Estimated moves: 10

Step-by-Step Solution

1. Start with the numbers that are already at their maximum. A number equal to the number of available adjacent cells means all those cells must contain bulbs. In the opening grid, examine the edge numbers first because they have fewer neighbors and therefore fewer possibilities.
2. If a number has already reached its required count of adjacent bulbs, every other neighboring cell must be unlit. Mark those cells as safe or dark.
3. Work through the grid systematically: place a bulb, update the counts of all adjacent numbers, and then re-check which numbers are now satisfied. Propagate your conclusions across rows and columns.
4. Remember that bulbs shine horizontally and vertically until they hit a red wall. Two bulbs cannot be placed in the same row or column without a wall between them. Use this rule to eliminate invalid positions.
5. If you place a bulb where it causes a conflict — such as shining on another bulb or making a number exceed its count — a red X appears. You have five attempts total, so think carefully before each placement.
6. When every number is satisfied and every white tile is lit, a green checkmark appears. The next round loads with a larger grid.

Strategy & Reasoning

The puzzle is a classic constraint-satisfaction problem. Each number constrains how many bulbs touch it, and the line-of-sight rule constrains where bulbs can go relative to each other. The optimal strategy is to start with the most constrained cells — edge numbers and numbers adjacent to red walls — because they have the fewest possible configurations.
Alternatives like random placement fail quickly because the line-of-sight rule means most random bulbs immediately conflict with each other. The already-lit green cells are fixed; you cannot change them, so treat them as given information and work outward from them. The level is designed so that every placement is forced once you apply both rules consistently — there should be no guessing required.

Tips & Common Mistakes

• Do not forget that bulbs shine across entire rows and columns until a red wall stops them; a bulb placed far away can still shine on another bulb.
• Do not place a bulb adjacent to a number that has already reached its required count; that would make the number exceed its limit and trigger a red X.
• Do not skip the edge cells; corner and edge numbers are the most constrained and should be solved first, as they force the most conclusions.