Number Pyramid

Number pyramids are a favourite mental-arithmetic puzzle. Each brick holds the sum of the two bricks directly underneath it. Some bricks are filled in; find the rest. Every puzzle can be completed step by step — whenever two numbers of a little triangle are known, the third follows.

1. Tap an empty brick and type its number on the pad or keyboard.
2. A brick equals the sum of the two below it.
3. Bricks that don't add up are shown in red.
4. Arrow keys or Tab jump to the next empty brick.

• Top-down subtraction: if you know a brick and one brick below it, the other is the difference.
• Bottom-up addition fills whole rows quickly once the base is known.
• Check as you go — a red brick shows the mistake straight away.