Ok so you start with -0.9. You need to find how much it will change in an hour and 30 minutes. So you subtract -0.9 from -0.9 and you'd get 0. Now, we have to subtract half of -0.9 (because 30 minutes is half an hour). Half of -0.9 is -0.45. Subtract 0 by -0.45 and you get 0.45.
