速算：99 乘 22 = ？

99 x 7 = 693;

99 x 15 = 1485;

99 x 87 = 8613;

99 x 36 = 3564;

...

99 x n:

n - 1 i.e.: 7 - 1 = 6;

100 - n i.e.: 100 - 7 = 93

so: 99 x 7 = 6, 93 > 693;

n - 1 i.e.: 15 - 1 = 14;

100 - n i.e.: 100 - 15 = 85

so: 99 x 15 = 14, 85 > 1485;

...

challenge:  

99 x 123 = ?

99 x 221 = ?

99 x 383 = ?

and how?

 

enjoy!