
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 

2.  Subtract the number from the nearest power of ten (10^{n}), as the second part of the answer.  In this case, the nearest 10^{n} to 99 is 100 And, 100  15 = 85 

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(10^{n}  1), where 10^{n} is nearest = 10^{n}x  x = 10^{n}x  10^{n} + 10^{n}  x = 10^{n}(x  1) + (10^{n}  x) Note that, since the results are 'joined', 10^{n} is always adjusted as the sum. 
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