This is the way I would do it:
The sum of the 1st & 100th numbers = the sum of the 2nd & 99th numbers = the sum of the 3rd & 98th numbers all the way to the sum of the 50th & 51st numbers; each of the sums equals 200. So I would multiply 200 by 50 (10000).
Their sum is 10000.
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.
The sum of the first 50 odd numbers is 2,500.
There is a surprisingly easy formula for this. Sum of n odd numbers = n2 So the sum of the first 600 odd numbers (starting with 1 as the very first odd number) is 6002 = 360000.
Their sum is 10000.
The first 20 odd numbers are: 13579111315171921232527293133353739.
The first 100 odd whole numbers are: 13579111315171921232527293133353739414345474951535557596163656769717375777981838587899193959799101103105107109111113115117119121123125127129131133135137139141143145147149151153155157159161163165167169171173175177179181183185187189191193195197199.
The first 85 odd numbers are: 1 3579111315171921232527293133353739414345474951535557596163656769717375777981838587899193959799101103105107109111113115117119121123125127129131133135137139141143145147149151153155157159161163165167169.
The sum of the first 300 odd numbers is 90,000
The sum of the first 1,000 odd numbers is 1,000,000.
The sum of the first 40 odd numbers is 1,600.
The sum of the first 50 odd numbers is 2,500.
The sum of the first 153 odd numbers is 23,409.
The sum of the first 8,000 odd numbers 64,000,000.
The sum of the first 15 odd numbers is 225.