Question

How many different seven-digit numbers are there the sum of whose digits is even?

Answer 1

Total no of $7$ digit numbers =$9\times {10}^6$
Without loss of generality,
We can safely assume that half of them will be even and other half will be odd.
so,
Total no of seven digit numbers having sum of digits even =
$\frac{9\times {10}^6}{2}$

= $4.5\times {10}^6.$ hence this the answer.