how to combine multipe if's in google spreadsheet

I made a single formula and use 4 cells, but I want to combine then into 1 cell. Everything I tried didn't work. So I hope you can help me out please.

My problem to solve is how do I get the 4 single formula combined into cell B12 only. Without using the other cells B13,B14,B15

=if($A$12<B23;B23*C23)             
    =if($A$12<B24;B24*C24)
        =if($A$12<B25;B25*C25)
            =if($A$12<B26;B26*C26)

http://ift.tt/28KjXFP

with kindly regards