<>it is not an essay, but a normal problem. i give u a link <a href="http://www.mathematik.tu-darmstadt.de/Math-Net/Lehrveranstaltungen/Lehrmaterial/SS2004/MatheIV_MB/loesung08.pdf" target="_blank" >http://www.mathematik.tu-darmstadt.de/Math-Net/Lehrveranstaltungen/Lehrmaterial/SS2004/MatheIV_MB/loesung08.pdf</A></P><>G27. ein modell des vergessen (a model of forgetting)</P><>though it is in german, u can understand the general meaning from the equation.</P><P>generalling meaning of the problem: a student have learned some knowledge, but he will forget it. please give the foregettng rate out. </P><P>try u best. and hope it will be helpful. but if u cannot understand it, then just forget it. </P>