just add them 1+3+5+7+9+11+13+15+17+19 = 100
or a trick you could use is realize that
19 + 1 = 20
17 + 3 = 20
15 + 5 = 20
13 + 7 = 20
11 + 9 = 20
20 x 5 (total pairings or half the numbers your adding) = 100
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The average is 10. You will find that the average when adding together the first n odd numbers is always equal to n.
The first 20 odd numbers are: 13579111315171921232527293133353739.
The first 85 odd numbers are: 1 3579111315171921232527293133353739414345474951535557596163656769717375777981838587899193959799101103105107109111113115117119121123125127129131133135137139141143145147149151153155157159161163165167169.
The sum of the first 1,000 odd numbers is 1,000,000.