Function only updates when saving

I have a problem with my Excel,
Column C has a formula =-(D4-E4)*80/1000
Column D and E will be given with values, but Column C can not automatically updates when entering a value to cloumn D & E..
eg. Row # 2, Column C has no value, given the fact that it has a formula.

What's happening to my Excel?
Please help..

Excell formula error.jpg

A B C D E F G H
Date Day POWER WATER

1 Fri 0.800 11925 11935 0.70 12973 12980
2 Sat 0.000 11936 11945 0.00 12981 12989
Sun 0.000 0 0 0.00 0 0

You may have Calculate set to Manual. Try this:

1. Click Tools and select Options.

2. Select the Calculation tab.

3. Under Calculation, select the radio button next to Automatic

You have a rather old version of Excel that I no longer have access to so I can't confirm this for you.