Since no additional information was provided, I'll treat this an a significant figures problem, since that's the only way this question can make sense.
Let's say you have to add a number to 0.259 in order to get 0.51. Here's how you'd go about doing that.
Notice that you need to go from a number that has 3 significant figures to a number that has 2 significant figures. The only way to do that and get the values given is to add a number that has only 2 significant digits, both after the decimal, to the first number.
When you add or subtract decimal numbers, you go by the number of sig figs that are after the decimal only.
If you subtract 0.259 from 0.51, you'll get
Notice that the answer has 3 sig figs after the decimal: 2, 5, and 1.
This means that you have to round this number off to only 2 sig figs after the decimal, which will give you 0.25.
Now, if you add 0.25 to 0.259, you'll get
Rounded to 2 sig figs after the decimal, the answer will be