Calculating the interest on a series of loans
Do not represent monetary amounts as a decimal type; use an integer type instead.
Try to avoid the use of using namespace std.
You could still allow the user to enter an interest rate and have the program convert it to a decimal, especially if the user doesn't know how to do this.
You could just have days, rate, and interest declared or initialized within the loop.  It's usually best to have variables kept in as close scope as possible.

Moreover, if you're going to "list" variables, have each one on a separate line.  This is easier to read and avoids the possibility of needing to scroll horizontally.