Whatever the initial number of people in the bus was, we know that this number underwent the following transformations: after the first stop, we have
[tex] x \to x-15 \to (x-15)+9 = x-6 [/tex]
After the second stop, we have
[tex] x-6 \to (x-6)-4 = x-10 [/tex]
So, globally, there are 10 people less at the end.