Math Is Fun Forum

I work in school and am having trouble working out some percentages.  I have 203 hours that are split between a number of teachers throughout school.  One of the teachers is leaving so another 20 hours are available.  I then have some children leaving for secondary school so 51.5 hours will be lost.  I know that the equation to work out the percentage of hours that each teacher will be losing is 230+20/51.5 which comes to around 23%, however a number of my colleagues think that the equation should be 203/51.5-20 and I cannot explain to them why they are wrong?  Can u help?

Any help would be much appreciated,

Thanks,

Julie Elliott