Which subnet is host 20.100.55.3 /26 in?

Use the Subnetting Secrets Chart© again.

1. How many subnets?

2^18 = 262144

2. How many hosts per subnet?

2^6 – 2 = 62

3. What are the valid subnets?

256 – 192 = 64

20.0.0.0

20.0.0.64

20.0.0.128, etc.

20.0.14.192

20.0.15.0, etc.

4. What are the valid hosts per subnet/broadcast?

The subnet values need to be added to the second, third, and fourth octets.  The last octet must be multiples of 64, and 0 is permitted as long as there are bits turned on on the second and third octets.  Below are the starting subnets.

Subnets	20.0.0.0	20.0.0.64	20.0.0.128	20.0.0.192	20.0.1.0
First host	20.0.0.1	20.0.0.65	20.0.0.129	20.0.0.193	20.0.1.1
Last host	20.0.0.62	20.0.0.126	20.0.0.190	20.0.0.254	20.0.1.62
Broadcast	20.0.0.63	20.0.0.127	20.0.0.191	20.0.0.255	20.0.1.63

And more subnets:

Subnets	20.100.54.128	20.100.54.192	20.100.55.0	20.100.55.64	20.100.55.128
First host	20.100.54.129	20.100.54.193	20.100.55.1	20.100.55.65	20.100.55.129
Last host	20.100.54.190	20.100.54.254	20.100.55.62	20.100.55.126	20.100.55.190
Broadcast	20.100.54.191	20.100.54.255	20.100.55.63	20.100.55.127	20.100.55.191

We can continue counting up but to get to the subnet containing host 20.100.55.3 would take some time.  It is to be found in subnet 20.100.55.0.

Subnet 20.100.55.0 hosts 20.100.55.1 – 20.100.55.62 (broadcast = 20.100.55.63).