Stuck! Please help with formula

[mcrubbin]

Hi guys!

I'm taking a beginner Excel Class. I need help!

In my Excel file I have a column full of numbers. At the end of the column, I have a SUM Function so it adds all the numbers together to create a total.
Now my book is asking me to create a 'complex' formula that will show a 30% decrease in the total number of cupcakes.

I tried:
=B9-0.3
=B9/.3
=B9*.3
=B9+.3

Btw, B9 is 172 in my excel file in case you want to try it.

but none gave me the answer I'm supposed to get, 120.4. I did some basic algebra on a piece of scratch paper and realized that I have to multiply 172*.7 and I get 120.4 but why?