wrong calculation on invoice of a project.

invoice are made on the basis of the timesheet activity. Here my product is service and unit price is 75 unit of measure is hours. So i create an invice for the tasks it shows wrong calculations Eg i spend 4hrs on one task then i will get an invoice like this 4 * 75=300 But now i will get like this 300 * 4=1200 Ie here unit price is calculated automatically as 300(75*4). How can be clear ths error in the project task invoice.