If the 1st fastest dog is 10 years old, and the 2nd fastest dog is 2 years younger than the first fastest dog, the 2nd fastest dog is 10-2=<<10-2=8>>8 years old.
The 3rd fastest dog being 4 years older than the 2nd fastest dog, is 8+4=12 years old.
The 4th fastest dog is half the age of the 3rd fastest dog, meaning the 4th fastest dog is 1/2*12=6 years old.
The 5th fastest dog is 20 years older than the 4th fastest dog, meaning the 5th fastest dog is 6+20=26 years old.
The total age of the 1st and 5th fastest dogs is 26+10=<<26+10=36>>36 years
The average age of the 1st and 5th fastest dogs is 36/2=<<36/2=18>>18 years old.
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