This can be thought of as a dimensional analysis problem. You know how to relate ml into doses so this is what the conversion looks like:

(100ml/1)*(1 dose/ 2.25ml) = 44.444 doses

Another thing to know while answering this question is that a fraction of a dose does not count as a dose, so you round down to the nearest whole integer. So, you can give a total of 44 doses.