A fast calculator also checks fast. Add the digits of a number again and again until one digit is left — its nine-check number. Do the same sum with the nine-check numbers. If the result does not match the nine-check number of the answer, the answer is wrong.
Work it out in your head or on paper, then tap your answer — or type it and press Check. Stuck? Press “Show me how”.
Right: 0 · Tried: 0
🏁 Lesson 13 test
20 questions. Check each answer, then go to the next one. Get 16 or more right to pass — and beat the clock: finish within 4 min.
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🔍 Nine Detective badge when you pass — and a certificate when you finish all 5 lessons of
Part 3 · Speed dividing, percentages and checking. You can still take the test without signing in.
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