How many 20-digit numbers are such that the sum of their digits is even?

Half of them will have digit sums that are even.

And half of the number of 20 digit numbers =

(99,999,999,999,999,999,999 - 10,000,000,000,000,000,000 +1) / 2 =

45,000,000,000,000,000,000

Thanks Chris :)

You know I can never remember this formula off the top of my head :(