Number Problem

Using following numbers and following operators you have to produce 28.

Numbers are 1, 5, 7 and 8.

Operators are +, -, * and /

You can use ( ) brackets as well.

Rules:

1-You can use each number only once.

2-You can not use any other operator or symbol.

3-You can not use 1 and 5 as 15.

4-You have to use all of the above number mentioned (1,5,7 and 8)