how to tackle this route summarization qs?

[Hondabuff]

The faster way with no Binary Math is: This will blow your mind how easy it is.

1) Find the Block size to cover all of the networks in the 3rd octet= Networks 0,1,2,3 answer is 4 block size.
2) Now count on your fingers backwards: 128,64,32,16,8,4,  Should have 6 fingers sticking up.
3) How many common bits in the first 2 octets, 8+8= 16, Now add your like bits from your finger counting 16+6=22
4) Answer is 10.0.0.0/22
5) Again, no binary math and it all works from your 128,64,32,16,8,4,2,1 chart

For example: Summarize these networks?

172.16.16.0
172.16.17.0
................
................
172.16.25.0

1) What block size covers the all of the 3rd octets? Answer is 16
2) Count backwards on your fingers: 128,64,32,16
3) 4 fingers are up, added to the first 16bits. 8+8+4=20
4) Block size started at 16 so the answer is 172.16.16.0/20