### Express $25$ as a sum of odd natural numbers.

$25$

Step by Step Explanation:
1. We know that
Sum of $n$ odd natural numbers = $n^2$

Also,
$5^2$ = $5 \times 5$ = $25$
2. Thus, we can write $25$ as a sum of $5$ natural numbers.
$\implies 25$ = $1 + 3 + 5 + 7 + 9$