Find the missing number

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Read the equation and type the number that fills the blank. Press Enter to check it. Press Enter again for the next item.

To find the blank, do the opposite operation to the known result. If a × [ ] = c, then [ ] = c ÷ a. If a ÷ [ ] = c, then [ ] = a ÷ c. With brackets, work from the outside in.

In Japan this is called 四則逆算. It is the largest section of the 玉手箱 aptitude test.

A bar empties on each item. When it empties the item is marked wrong and the next one appears. The real exam runs at about 10 seconds an item. Levels 1 and 2 give you 15 seconds and levels 3 and 4 give you 20, so you can learn the method first.

Level 1 uses one operation, and the missing number is up to 20. Level 2 raises the missing number to 100 and can put the blank on either side of the operator. Level 3 adds a second operation inside brackets. Level 4 uses a decimal or a fraction.

Your score is how many of the ten you answered correctly. It is arithmetic practice, not a measure of ability, and it is not scored the way the real exam is. Finish all ten items to save the round in this browser. Ending early saves nothing.

Type digits, and a decimal point if the answer needs one. Enter checks your answer. Enter again moves to the next item. Escape ends the round.