When multiplying by 9, multiply by 10 instead, and then subtract the other number. For example,
23?—9 = 230 - 23 = 207.

More examples and explanation

The same applies to other numbers near those for which multiplication is simplified:

23?—51 = 23?—50 + 23
= 2300/2 + 23
= 1150 + 23
= 1173.

87?—48 = 87?—50 - 87?—2
= 8700/2 - 160 - 14
= 4350 - 160 - 14
= 4190 - 14
= 4176.