Change var depending on dropdown list index item JS

I'm trying to do a loan calculator where you can stimate the monthly playment value according to diferent interest and secures.

everything is working fine now, but then I was told that my var X variable depends on the type of credit.

that beign said:

var X = Y*0.00342

BUT

if items # 0,5,6,10,16 are selected in drop down list

THEN

my var X becomes

var X = Y*0.00042

so my question is:

How can I make this 'var X' to be depending on dropdownlist items?

how do I integrate this condition inside the function?

thanks in advance! :)