Steps | 15 × 99 | ||
1. | Subtract one from the number. This is the first part of the answer. | In this case, we get: 15 - 1 = 14 |
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2. | Subtract the number from the nearest power of ten (10n), as the second part of the answer. | In this case, the nearest 10n to 99 is 100 And, 100 - 15 = 85 |
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3. | Join the results to get the answer. | In this case, Joining 14 and 85, we get 1485, which is the answer! |
24 - 1 = 23 And, 100 - 24 = 76 Thus, 24 × 99 = 2,376 |
356 - 1 = 355 And, 1000 - 356 = 644 Thus, 356 × 999 = 355,644 |
878 - 1 = 877 And, 10000 - 878 = 9122 Thus, 878 × 9999 = 8,779,122 |
For any number, x × 99... = x(10n - 1), where 10n is nearest = 10nx - x = 10nx - 10n + 10n - x = 10n(x - 1) + (10n - x) Note that, since the results are 'joined', 10n is always adjusted as the sum. |
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