Simple total differentials

I need to take total differential of the following:

$M/P = m(y/r)$
$M = s(k,r)B$

I got something like this:

$d(M/P) = ?$
$dM = s_k dk + s_r dr + dB$

( $s_k$ simple means the partial derivative of s with respect to k.)

I then need to find out $dr/dk$ and $dr/dB$ but I can do this by myself.