december 2014 q 1

Hi Dear Tutor, I have a question. Cum-div=ex-div+about to be paid=3.7107=3.10+0.6107 then we deduct 0.21 from 3.7107 which is 5.50$ here is 3.10$ is ex div and about to be paid is 3.10*0.197=0.6107 about to be paid is future pay-0.6107 has just paid is past pay-0.21 my question here is that why we deduct past oay from cum-div to get current share price?it should be past share price not current smth seems to be confusing thanks in advance