How to sum 3 or more results in 1 cell

Hi dears,

This thing stuck me up..may be i wil be little for you guys..bue very hard for me at the moment..

I have Cell A1 and B1..

Cell A1 is for Number of Days..
Cell B1 is for an amount which is based on days..

I have 3 kind of Rates..like..days from 1 to 10 is 3 USD
Days 11 to 20 is 6 USD and Days 20 to onward is 12 USD.

Now what I want if Cell A1 has days "5" then B1 should give me result 5x3 = "15", but if A1 has days 15 than B1 should give me result "60" because first 10 days (10x3) and remaing 5 days (5x6) total 30 and as well as if days are more than 20 than same practice..

How i solve it..

Thanks in advance for your costly time and help.