To add these fractions, we first need to find a common denominator. The common denominator for 400 and 520 is 520.

So, we rewrite the fractions with the common denominator:

325/-400 = (325 * 520) / (400 * 520) = 169000 / -208000
-286/-520 = (-286 * 400) / (-520 * 400) = -114400 / -208000

Now, we can add the fractions:

169000 / -208000 + -114400 / -208000 = (169000 - 114400) / -208000 = 54600 / -208000

Therefore, the sum of 325/-400 and -286/-520 is 54600 / -208000.