How do I calculate the value of b from this formula?

I have the following forumla:

(((a*(1/b))-1)/(a-1))*1000 = c

But I need to retrospectively go back through some data to find the value of b, which was not recorded (a and c were). How can I do this?

>Solution :

((c/1000)*(a-1)+1)/a=1/b

then

b=a/((c/1000)*(a-1)+1)