You have address 200.100.20.0 /24 and the client wants to break this down to at least nine subnets each having at least 10 hosts per subnet.

Here you need to generate AT LEAST nine subnets but not waste subnets.  Tick down the powers of 2 subnet column until you reach the closest number possible to nine without wasting subnets.

	Bits	128	64	32	16	8	4	2	1
Subnets
128	√
192	√
224	√
240	√
248
252
254
255
Powers of 2	Subnets	Hosts - 2
2	√	√
4	√	√
8	√	√
16	√	√
32
64
128
256
512
1024
2048
4096
8192
16384

The closest we can get is 16 because 8 is not sufficient.  Tick down four places on the upper subnets column to generate a subnet mask of 255.255.255.240.  You have taken 4 bits from the last subnet leaving 4 for hosts.

16 – 2 gives us 14 hosts per subnet which is sufficient.