I'd like to be able to create a cell which is the sum of the values in column A, but only if the value of column B in the cell beside the value in Column A is equal to some number (like = 12).

So that for this

col A col B
3 45
30 12
34 12

Sum of Column A where column B is also =12 would be 30+34 = 64

(I'd end up wanting a bunch of cells like this, using different numbers instead of 12, ie another cell that has the sum of column A where column B is =11)

What's the best way to do this?