How are the Utilities unmortgaged with 10% interest if the mortgage value is $75?
There is no official rule to address this question, so you and your play group must decide how to handle it.

Note that the same question applies in the case of landing on the Income Tax square and choosing to pay 10% of your worth.

A common thing that would make sense is to follow the same rules of rounding that is taught in most basic math... 1/2 and higher means round up, lower than 1/2 means round down. So 7.5 would round up to 8.

But if your group decides on either always rounding up, or always rounding down, that would be equally legitimate. 

However, even if you want to use mathematical rounding, there are actually different conventions for handling rounding 1/2... all conventions agree that less than 1/2 means round down, while more than 1/2 means round up. But for 1/2 exactly, there are multiple options.

Because Monopoly is a game designed to simulate handling money, involving banking and such, it might make the most sense to use what is called "Banker's Rounding", which is also known as "Round to even". This means that 7.5 would become 8, but 10.5 would become 10. In other words, when dealing with 1/2, round to whichever direction would leave you with an even number.